Maybe I did not fully understand everything, but I think there has to be something more to it. Check the latest save. Hurl can do anything now without being interrupted in player's turn, even stand one tile from Santino. He can also move out of the room and not be interrupted in enemy turn either. Now if he walks into the room in the next turn (even southwest), he will be interrupted. But if he waits until Santino is NotKnown again, he can enter the room southwest (only) without interrupt (now there is a semi-freeze, you need to press ESC), then do whatever he likes, again. It does not have to be 1AP turn, just whatever.

I used to rely on a dropped ammo to know it.

Yes, thats one that make me realize about Hearing interrupt work other way for Santino. I managed to reproduce it by moving into Eastern door diagonally to South-West, so Santino seen merc, but merc didnt seen him. For regular merc it would be free interrupt, as Victim didnt seen Interrupter, but Santino have a special rule to not see if he not seen too by that merc. Then i shot door (could be any other target, as long merc not turn to see Santino prior shot), and got interrupted by him via Hearing detection, its very short glimpse, as Santino will do nothing if he's still unseen, and will immediately pass turn back. After that for this turn Santino cannot interrupt again, as merc is CurrentlySeen, and will act only during own turn, trying to detonate bombs after speech, but due to bug (i fixed that one already), some bombs will not explode (this one is about his action during Enemy turn, it doesnt matter if its caused by interrupt of player interrupt by him, or by failing to act during interrupting in player's turn). In your save, Santino got an ExtendedEars (and M16 rifle, eh, so they grabbed some Shipment as i got), and can hear Hurl at tile where he's standing. In my tests i had to shoot to let him hear merc, but overall its the same.

This is one of oddities about Own's List, it let mercs see others who are behind them:
As Santino become SeenLastTurn and then SeenTurnAgo for Hurl those turns you're about, Hurl do see him while moving South-West from door (despite not actually looking the direction to Santino (NW)), enabling Visual detection for both of them, and thus interrupt for Santino. This could lead to seemingly weird stuff with mercs who merely heard eachother prior (say, merc, who only heard another merc firing a gun in other room behind a closed door and never actually seen him yet, will see that merc with own back from a whole screen size distance if the shooted merc will open a door to his room). Turn after that, Hurl forget about Santino, and it become NotKnown in that list, so movement not cause detection and interrupt, as in first time he entered that door.

Thats purely brilliant find, i searched for this particular bug for very long time, but was unable to get a reproducible save, immense help man, many many thanks, you're amazing. It was easy to fix with such handy save in disposal.
First of all try to move not to that closest tile near door (where Hurl is standing in your save), but click to the one next to West from it. Youll see that not only no freeze happens, but also they will see eachother without interrupt. Non-buggy part of it is similar to described above - Santino hear Hurl, who not see him, so interrupt happens, but not cause a speech, and when Hurl also see Santino after moving to the next tile (or just whatever) - he is already CurrentlySeen for Santino, so no interrupt is happens.
Now to buggy part, as its of not really banal circumstances, it was elusive for long. Due to bug, game "forgot" to finish a movement for a merc, who is interrupted by Hearing during move to the exact tile that is destination for that merc. If that tile is a some tile merc is just passing by, as with movement to more Western tile there, no problem occurs. And as Hearing interrupts rarely caused by moving at all, its even more rare to get them exactly on a destination tile (unless player move mercs square-by-square its a real luck). Pressing ESC make merc abandon the current action, but as merc is actually already finished it here, it only remove "semi-freeze".

Was that intended and how does it make sense? If the fix was that he just interrupts on every enter, it would eliminate other problems. You already fixed the explosives during Enemy turn, but it still looks silly for the player to shoot at Santino and watch him swallow bullets or grenades.

Ok, so this one isn't actually related to Santino, but to hearing detection in general? And, as I understood, it would not happen if Santino did not have ExtendedEar? If so, it is easier to encounter this bug with them taking all shipments, as they might get ExtendedEars.

They drop (or they don't) ammo when killed, so that isn't very useful by then. What I meant is that it's good to know what the enemy is using against you during a fight (even before you see him), it does matter for strategy.

Im pretty sure its buggy, i wrote it.
I think they encountered a problem with Santino interrupts on hearing stuff going on in nearby rooms, and explode it without player mercs nearby, so they put a patch after patch on it (Santino not interrupt on Visual if merc not see him, yet still check if he is seen for a speech, so they knew its not enough, so added it). There is still at least one more way to make him stay there alive, not related to interrupts, i can fix it too, but it will prevent many fun stuff possible with that room, player can try to break it some new ways. Afterall nothing of it is ruinining game, even with "alive Santino" player will win the same way. And to make its an optional fix its too minor.

No, i managed to reproduce it, but its about merc doing particularly loud sound on moving on that tile, its possible, but rare, while with Ears its easily reproduceable. I encountered this bug even in S59 on first day, just i been unable to reproduce, its immence help, i think it was a most dangerous bug still present that im aware of.

Thats useful when you play next time. Im not say that sound is useless, but also im not sure how matter if some particular enemy who already shooting at your mercs have essentially m16 or merely m14 (btw, they have special sound also for modified weapons)? Id say important thing is metagame stull like "this black-haired guy in S48 have m14" or "that brown-haired guy in S27 have a detonator" (they could be not guys but girls, actually, as bodytype have no effect in game, and unlike hair colour is random for every entry to sector).

M16 vs M14 may not matter much, but M16 vs .38 would really make a difference.
Modified rifles do have special sounds, modified pistols stay unmodified (or I cannnot discern them).
I don't think memorizing guns by specific enemies is a worthy investment (who carries a detonator - more so) - they are shipment-dependent and "enemies nearby" are still undpredictable (really a lot of that in "go ahead").
As for sounds, I think they may be useful also in other situations, you can hear doors opening, for example.
Sounds suggest all of enemies are males, I didn't even know they have a bodytype (any of the 8 bodytypes that my mercenaries can have?) Maybe they have it just because they need to have all attributes.

In "normal" game (when player do not restrict himself with some own rules) enemies are totally outperformed after player got some access to guns reliably firing out of Camo detection range. GEAR helps with it ofc, as its not just 5 shots to kill anybody anymore, but still enemies are never shoot out of their personal view range. "Memorizing guns" is of use for early stages of game, like that m14 enemy in S38 (not S48 ofc), where its about only example of uncertainity "this enemy is with rifle or Browning?", and on later stages they all roughly use some rifles anywhere, as they all fire not farther than 14 tiles, those rifles are not of essential difference id say. In "normal game" shipments are just not exists, as they horribly delayed (i think they set them not realized about Armor bug), and reinforcement ones are pretty rare, GoAhead only let them appear in Day1 and increase a rate of department from S1. If modified pistol sounds the same as non-modified its more misleading then, id say. There is not much difference between M14 and modifiedM16, but difference between Beretta and ModBeretta is large. Doors and explosions are definitely useful, are door opening heard over whole map, or it have to be in some range to be heard? Sometimes i have to set to Map view on sabotage missions to check if they triggered it, should be easier with sound there for sure.
Bodytypes are used only for a picture in Inventory, thats all, but they still assign them (for non AIM mercs & Guides they are just random). They do use some 8 values for it, actually i thought there is only about 5 of them, so i dont know if some are duplicates or not. At least there were no bugs with them like that with hair colour (see attach).

As for "normal" games, you are generally right (it's just that as I maximize diffiuclty, I usually speak from that point of view).
M14-mod.M16 and Beretta-ModBeretta: I would disagree, the differences are similar (a look on the table you posted would confirm that), especially if you wear treated SpectraShield.
I'm not entirely sure about the doors, I guess some range is indeed needed.
There have to be 8 bodytypes, as I recall them all: tall, average, short, fat, the MonsterTruck, tall female, petite female, the Stella.
The attachment does not work.
Reinforcements rare? I don't think so. For instance, on day 2 I entered S49 and got 3 reinforcements to that sector. After I killed them all, the day was coming to an end, so that was it for day 2.

Ms all will be fired inside their range, as AI never will do out of its view. Regular Beretta will be out of range at some distances, and will also be very weak there, while ModBeretta will do still fine. Its pretty big difference, id say.
I never even knew there so many types used. (upd i checked them and i think i got why they dropped them from being a pure visuals: some of them are nearly identical)
I see, and i dont know what to do with it, i recall some problems with it in past, but also i managed to post some somehow. Now it accept only .bmp from me, and even then it doesnt work. Its nothing important, just screenshot abvout curious hair colour bug.

Thats pretty rare, yes. Also its not any change from Hard (actually even Easy) difficulty, you could get 3 attacks on a sector at day 2 there too. Even more than 3. I just mean there is not so important to hear the sound of their guns, ideally they shouldnt shoot to mercs at all, as they could hit a legs and no armor will help (surely in games about speed there is a need to play more agressively after getting good armor, but its still not "just stay under bullets, as they fire only handguns" (maybe i got you wrong or so)).

You cannot attach a jpg? I did with no problems. 1.bmp still leads to nothing.

No, sound FX are not of critical importance. What I'm saying is they may be helpful in general and are not useless. So I did not get why you chose play without it. That would lessen the experience for me, I just mute some mercs whenever they start to get on my nerves. But that's just me, it's your choice.

No, it say "bad image format". (upd: oh fuck, they bother for uppercase, .JPG is a "bad image format", amazing).

I dont say they are useless, just i never bothered with them. Playing with sound will make a nuisance to people around, and if its not a issue id better put a some music on instead. That means what i dont know about bugs related to sound, if any (except that crappy one that happend essentially with sound off and resulted in outright crash to OS).

21. I have had some game freezes (hard ones, I had to restart DOSBox) at the event of enemy throwing tear gas. I don't think I have experienced this on my latest game in 1.15y. Maybe I just have been lucky with it this time, maybe it has something to do with core parameter (I changed to normal for this version) or maybe you fixed it already (but I couldn't find anything related to it in readme, unless it is this one "Occasional lockup caused by AI's routine resulting in game freeze during enemy turn is fixed").

You mean you had it on some 1.15 pre-y? Or in unpatched version? I think there been something about AI entirely stalling indeed, but not something particular to tear gas.