This just happened to me again this morning. First project I am working on for the day. Computer was not rebooted. Just logged in and opened Premiere. I know you guys know what the UI looks like but just wanted to paste these here anyway.
No sound at all. Why? No reason. Nothing else is muted or Solo'd:
Clicked Solo on Audio 2. Got sound on audio 2. Then unclicked Solo. And now all sound works as expected.
Why?
No reason.