...
Ok, once again a very old game on GOG was unfortunately released with poor compatibility fixes (sorry, but that's the truth). In particular NOX from GOG did:
1. not fix missing music while in game (because there is some music https://www.youtube.com/watch?v=0OXvIj8l5Sk )
2. force the game to start in Win 98 mode and with unnecessary and potentially harmful Admin rights (bad habit abused by many newbies)
3. still have flickering/freezed screen layers issues for some users

THIS is the appropriate fix (see attached pic flags.jpg)
You'll need MS ACT from http://www.microsoft.com/en-us/download/details.aspx?id=7352 ~20MB, you don't need to download and install full 6GB package, don't worry! For Windows 10 click at the bottom Related Resources > Windows Assessment and Deployment Kit (ADK) for Windows 10.

...and you won't need to start the game as Admin or using 3rd party hacks and workarounds. It will solve also flickering screen on some systems, and you'll get also music back during play (please restart a new game!).

Tested on Windows 10 x64 (with retail CD and GOG version, nVidia 353.84 GTX 980M; AMD 7970M Crimson 15.30.1025)
Enjoy! ;-)

P.S.
For those newbie totally unaware of what is and how to use MS ACT I suggest this useful guide I've found for you on the Net:
http://classicgamefixes.wix.com/home#!act-toolkit/c19h5

Detailed help for some of the ACT shims can be found here:
https://technet.microsoft.com/library/dn383989.aspx?f=255&MSPPError=-2147217396
https://technet.microsoft.com/en-US/library/cc765984(v=ws.10).aspx

Once created a fix, it will appear as any other "installed" program for Windows so it's easy to remove, in case.

EDIT3 (Oct 2020):

First of all, remember to REMOVE ANY COMPATIBILITY SETTING (Run As Admin, Win98 compatibility mode etc.) you may have set before for "game.exe"!
This is crucial or you'll still have flickering issues etc.

Recommended shims for "game.exe" to fix music, flickerings etc. are:

Compatibility modes:
8And16BitAggregateBlts
DXMaximizedWindowedMode

Compatibility fixes:
8And16BitTimedPriSync
DirectPlayEnumOrder
DisableThemes
DPIUnaware
EmulatePlaySound
NoGDIHWAcceleration
Win98VersionLie

(See attached picture)

EXTRA HINT:
You can play this game in full screen 16:9 without black bands setting a 1024x576 resolution in "nox.cfg":
VideoMode = 1024 576 16
But before you do this, you have to add this custom resolution to your available screen modes of course, for example through the NVidia Control Panel: Display > Change resolution...
For 16:10 you can try 1024x640, and for 21:9 maybe 1029x441 should work.

People, ignore this post. The SDL patch is a much better solver than a compatibility kit, considering it literally upgrades nox instead of downgrades your OS.

You don't know what you're talking about. The OS is not downgraded, it acts on single exe. It is the official tool from MS BTW to mitigate compatibility issues, exactly same thing many Windows updates etc. do. You need ACT just to create and apply the patch to the system. You can uninstall it once finished. Windows internally already handles those compatibility layers set by ACT. If you think you need a complicated patch tool (this ATC) to be running while playing you're totally wrong...
You don't believe me? Take a look at your
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers
registry key for example...
It's the cleanest and more retro-compatibile settings you can do in Windows, but you're free to use 3rd party hacks and unnecessary tools and files that may not work on all configs if you care...
Does this SDL patch fix also music while playing for example?

You might be forgetting that NoX is really old. Anyway, of course the SDL patch fixes music and video and all.

Why should I download and install some toolkit if I can just install SDL patch which takes literally 1 min and the game works flawlesly. And yes, even the music works.
You should have tried it before you started arguing about that.

PS: I have tried that MS ACT toolkit and had no idea what to do. I guess I would have to read freaking manual to handle it.

No offense but your advice is pointless in this case. It might be useful for other old games for sure but not for Nox because there are very smart people who have already handled those Nox compatibility issues. There is a whole community keeping the game still playable if you didn't know.

But do you understand what ACT is and what MS Windows compatibility layers are in the first place?
The standard built-in by MS Windows to fix apps incompatibilities used in any windows update and already in everybody's Windows installation...
When you select from Properties/Compatibility Mode/Run as Vista SP2, or as Admin etc. you are actually using them (you can even browse through these preconfigured modes with ACT, as they are just collections of fixes grouped together...). For this simple reason is the best and cleanest way on Windows to fix exe incompatibilities. ACT is just a tool provided by MS to create and manage them.

Learn how to use ACT and you'll be able to fix many old games incompatibilities without the need to wait for magic patch-fixes from other users on the Net or any other hack, if any available...
No need to run everything as Admin (risky!) and install hacks and 3rd party wrappers etc.

May seem hard the first time you open ACT interface, but it's quite simple if you read the guide example I posted at the bottom of 1st msg.

I prefer standard clean Microsoft way to fix exe incompatibilities rather than using 3rd party wrappers and dll injectors (technically they are hacks) and additional files to find and install, not always working with all HW/SW configs. I could pass it to MS itself, and Nox will magically be working fine with next Windows update for everybody..

Hope this thread can be useful for other users.
Bye

Actually what you're saying isn't wrong, it just doesn't really work for NoX. Compatibility fixes do fix windows related issues with NoX but not graphics card related issues in this case, because the compatibility would have to come from nvidia itself, not windows.

I personally prefer this fix to SDL, as SDL is stupid when it comes to screen resolution and stretching.

You have no idea what is Nox SDL patch.

SDL is used in many other games and dosbox. I have seen enough of it to know not to bother with this one.

Updated in the first msg 'flags.jpg' for best results on ANY gfx card AMD and nVidia. In particular no more flickerings on some combination of gfx card/drivers due to GDI HW acceleration.

Sorry you're totally wrong. They DO fix also gfx issues, indirectly. Go learn what Windows compatibility layers/shims are.
The problem is that most users have no idea how Windows/drivers work and what ACT is, and rely on unnecessary invasive and potentially dangerous 3rd pary hacks/wrappers, when all they needed was a pair of compatibility flags.
When you learn how to use them wisely you can fix also many other (old) games issues, thought "impossibile" to fix.
On Steam I've fixed also Max Payne 1 and 2 for AMD/nVidia gfx cards:
http://steamcommunity.com/sharedfiles/filedetails/?id=595311477
http://steamcommunity.com/sharedfiles/filedetails/?id=595340012

It's something especially GOG users (many retro-gamers here) should know about...

I appreciate you pointing this program out, but for me your settings didn't fix the flickering cursor issue I have for Nox in Windows 7 x64. Based on reading the linked tutorial, there would be no way to fix this without trying hundreds of different possible combinations, one at a time. And even then it's possible ACT doesn't even have a setting that would fix it anyway.

It's always good to have more than one option for fixing a problem but the SDL patch has never failed yet and does a perfect job on audio, videos and scaling, if you're paranoid about unofficial patches you can even inspect the code and compile it yourself.

It's completely open source: https://bitbucket.org/awesie/nox-sdl/src

With either fix you will still need to do some more work to be able to play online with the GOG version of Nox, all of which is detailed in this one-stop guide: http://nox.introverse.net/installguide/