I got it to work. It was as simple as installing the software that comes with the game (that Steam should be installing when first running the game, but isn't).

Here's the list of commands I used:

WINEPREFIX=~/.local/share/Steam/steamapps/compatdata/329380/pfx wine _CommonRedist/DirectX/Jun2010/DXSetup.exe
WINEPREFIX=~/.local/share/Steam/steamapps/compatdata/329380/pfx wine _CommonRedist/OpenAL/2.0.7.0/oalinst.exe 

Actually, the menu works, but the game is still a black screen.

I think this is related to OpenAL. I've copied several OpenAL32.dll files from other places and pasted them into ~/.steam/steam/steamapps/common/Stealth Inc 2, and each one resulted in something different (sometimes the game doesn't start at all, sometimes the black screen appears after the initial logo, sometimes the black screen appears only when starting the game from the menu).

Proton 4.2-3, I just get instant ctd.
steam-329380.log

With Proton 4.2-3 the Launcher and game starts up. it shows the first two intro videos and then it ends in a black screen.

Log: steam-329380.log
Sysinfo: https://gist.github.com/NoXPhasma/f842039588ff89403b66abae978a4882

I got it working 100% on my system (Proton 4.2-9). Had to install DirectSound libraries:
WINEPREFIX=path_to_your_game_prefix winetricks dsound
Make sure to NOT use "PROTON_NO_D3D11". This caused that assets won't render correctly.
Just let DXVK do it's job. @kisak-valve might suggest to add this dependency to the game's installation script in Proton to make it (potentially) work out-of-the-box for ppl?

Proton 4.2-9 here as well, I can confirm adding dsound fixes the game. :) Thanks for the tip MadByteDE. :)

I can confirm that after installing dsound the game works fine, thanks for the tip @MadByteDE!

I also manage to get it working following the tips of @MadByteDE! , Thanks.
This are the steps that I've followed:

1 - Install winetricks typing in the command line: sudo apt install winetricks

2 - Run winetricks in the command line to install the DirectSound libraries: WINEPREFIX='(Steam-folder)/steamapps/compatdata/(GAME-ID)/pfx/' winetricks
You have to modify the command to match your personal 'Steam folder' and the 'GAME-ID'. Example: WINEPREFIX='/home/cobblestone/.steam/steam/steamapps/compatdata/329380/pfx/' winetricks

3 - Select Install a windows dll or component > dsound

4 - Open Steam > Stealth Inc2 Properties > SET LAUNCH OPTIONS and type PROTON_USE_WINED3D=1 %command%

5 - Enjoy the game

System Info: Proton 4.2-9 - Xubuntu 18.04

@kisak-valve
Does anybody know how to proceed now?. The game works fine but needs :

• Runtime config options : PROTON_USE_WINED3D=1 %command%
• DirectSound libraries(dsound) in the WINEPREFIX.
  Should I make a whitelist-request or is there any other request for this cases?

Hi @kisak-valve
Thanks for the answer. Unfortunately debugging wine is far beyond my capabilities. So I will leave it here, for someone else to take over. If anything else is needed, please let me know.

Just a quick update, the game appears to run flawlessly now with Proton 6.3-5 (dsound issue seems to be fixed). Only small issue is that my DS4 Controller won't work in-game.

System Information

• Steam Deck (LCD)
• Video driver version: 4.6 Mesa 24.3.4 (git-8d0948917e)
• Kernel version: 6.11.11
• Proton version: 1763652710 experimental-10.0-20251120

Closes itself right after launching, shows nothing (without adding anything through protontricks). No pre-launch screen shown. Proton log says: "Couldn't get first exception for process 0134 Z:\home\deck.local\share\Steam\steamapps\common\Stealth Inc 2\settings\settings.exe (WOW64)."

steam-329380.log