Thank you.
I did come across this fix and another one, but decided not to mess up my PulseAudio configuration, because it works just fine with normal WINE. My guess is that one of the patches Valve applied for Proton is a still a bit experimental. I’m staying with my own WINE version for now.
Update: i’ve tried some of the fix suggestions, but only one actually worked. Adding tsched=0 in /etc/pulse/default.pa fixed it for me.
...
### Automatically load driver modules depending on the hardware available
.ifexists module-udev-detect.so
load-module module-udev-detect tsched=0
...