So I have a external USB audio card (1st generation Focusrite Scarlett 6i6) and whilst not the most linux compatilbe card out of the box, I usually manage to make it work just fine. And indeed almost everything audio-related in my system works: I can play audio, sounds, songs, music streaming, youtube, I can even do audio production with my daw. Native games have no problem at all with sounds and score, but Proton games are completly silent. I thought the problem was with Proton but I have just tried running a game with Proton via the Heroic launcher and audio worked flawlessly. So the problem is Steam itself (with proton, native games are ok).

Do you guys have any idea how to force Steam to use right audio device?

Oh I almost forgot, I'm on EndevourOS with KDE plasma