Same here.
When I first plug in the headset, it sounds OK. When I start up SteamVR it goes full distortion - sounds like compression is wrong or something.
To workaround it, I have to right click volume control to go into Sounds (so I get a full list of devices) then in Playback I have to choose VIVE Pro Mutimedia Audio and then after that I have to choose Vive Pro.
That stops the distortion for me, but it absolutely sucks having to do this every time. Really shoddy work.