@pa4sam this post, in the reddit thread you linked, helped me:
Hey! I just had this problem and the solution was not obvious. I don't understand the whole complexity of it, but for me it was missing some libraries (notably d3dx9) that are not bundled into proton by default (I imagine because it is so old!).
To fix this I installed proton tricks following this guide (https://www.simpler.website/html/2021/1/protontricks.html) and then installed the d3dx9 library with the command
"protontricks 9420 dlls d3dx9"
I also needed to run
"protontricks 9420 dlls xact"
This was on a fresh install of Ubuntu 21.04 with nvidia 470 drivers. But i tried absolutely everything (nvidia drivers, different proton, reinstalled steam, even reinstalled Ubuntu, tried every 386 driver i could find etc) in every thread before this and nothing worked. Game crashed on launch. But i could start the game fine after these tricks.
Edit: if you change proton version, you need to redo this, (and maybe recopy the run file)
Edit 2: actually i seem to have no sound at all ingame...
Edit 3: lets give an update. I fixed the sound simply by changing "headphones out" to "line out" in Ubuntu sound settings. Dunno why it worked, i have never had issues before. Maybe something with xact and proton and gnome and pulseaudio... I use old analogue headphones, without USB connectors.
Anyway i found a spare drive and installed manjaro, fresh install steam and forged alliance, and get the same errors. Only installing d3dx9 and xact fixed them. Sample size = 2 but i think those libraries are missing from proton. (I also had the exact same audio problem in manjaro).