I have this same issue! I'm glad I'm not alone. Everything works perfectly until I start SteamVR.
All firmware up to date.
Using all original cables.
Sometimes a computer reboot or messing with the settings in windows sound settings will get it working, but not always. And I have to do that same song and dance every time. It has not worked first try for me in many weeks.
Did anyone find anything that works? I'm *this close* to just wearing earbuds under the built-in speakers on my $800 headset. Seems like a joke.