It's a known bug. I don't want to take the effort of digging it up, but there was a reddit thread about a month ago about this and someone linked to the steam forum post with tons of people having this issue and valve stating that they're working on a software fix.
Mine also does this. It's very annoying and sometimes I just plug in headphones while docked because of it. Other times I come out of sleep (specifically in persona 5) and the audio is weird and choppy, but I don't know if that's related.