What happened to multiplayer? The game document says
- 3 game modes: Campaign, Survival and Multiplayer

Where is Multiplayer?

This is actually a very good question. I recently bought this game here on GOG and I think a multiplayer mode will be overly fun. If this was in the original game, why not here?

good question.

I bought both games from GOG, and I don't see the multiplayer option in either of them.
Co-op would be so much fun :)

I've played Alien Shooter 2 co op campaign mode with a friend a long time ago, yet now, I can't seem to get it to work or find anything about it, yet I have vivid memories from playing through the entire campaign, over hamachi I believe.

You likely won't see this post, but the game has had two versions. One named Alien Shooter: Vengeance and one named Alien Shooter 2: Reloaded. The latter has less music, less skills to worry about(Each weapon type had its own skill, and Armor had its own skill as well), stats were altered to an extent, a couple of survival modes added, and two levels added.

Oh, and they removed multiplayer.

...Yup.

I forgot to check in here for a very long time, but now I have. That makes sense. I think it was vengeance we played then. Thanks a lot for the clarification, I had no idea they split it up into two.

They've also removed voice files, which should be played when someone contacts you via the radio. I remember getting them from the original CD version and putting into the "Reloaded" one. All this was done because, originally, the game was distributed on 2 CDs, because Internet connection at home wasn't something that most people had here in Russia (not to say, broadband internet connection with unlimited traffic), and CDs were still the main way of software distribution here, while, later, more and more people started getting the ability to get home internet connection, and became able to download software from the Internet, but data transfer speeds were still far from great, and the amount or monthly available Internet traffic was quite limited, so guys at Sigma decided that they need to make their game as compact as possible, so it could be distributed via online downloads better, so they've removed or simplified everything they thought was not necessary, and made the game simplier (most people saw Alien Shooter games as a small, quick, kinda, stress-relief alien grinder to quickly and mindlessly kill some free time, and got kinda confused when they've got almost and RPG with almost Fallout-like, complex character development system as a sequel, and not everyone liked that, so guys at Sigma decided to simplify that too, and, I guess, Multiplayer really was buggy and not really popular, so they've decided to get rid of that part of the game also.)

Music and voice over are returned some time ago... for Steam version? And increased size.

Also, russian retail of Reloaded have music (original version from website did not), but still dont have dubbing. Russian localisation! Without actually russian voice over!

(Yep, original language is English, and I guess same for all Sigma Team games).

Multiplayer still possible with retail versions of As2 Vengeance. I dont think different localisations are compatible.
Direct IP or LAN only. Some modes that removed, but Reloaded still have some new mods that is be good to try them co-OP. As I remember Vengeance have some co-OP- survival only. YT have videos of that gameplay.

Currently Sigma Team work for mobile (and Steam), some games released in past years. I didnt try them. Did they put some MP features to one of them?