Original reporter here. I now believe my initial problem was a hardware issue found in third gen Carbon X1s, not a bug in pulseaudio.
My original problem spontaneously fixed itself. I assumed that maybe the issue went away. It just recurred today. I found this thread on the Lenovo forums. https://forums.lenovo.com/t5/ThinkPad-X-Series-Laptops/X1-Carbon-3rd-Gen-Audio-Jack-Doesn-t-Work/td-p/2116099 The final comment in that thread suggests pushing down a connector on the motherboard. I gave it a shot and it worked, my audio is back. It appears that other people replied to this bug with their own audio issues. I'm not a Debian developer, but I think this bug should be closed. New issues might be created for the other problems, but I don't know if any of them have provided enough information. Thanks, -- Nate
