I noticed after installing some software, audio settings seemingly get changed out of the blue.
Went back to confirm all settings (computer line output [2 channel] was especially set very low), toggled between line and hdmi audio out (hdmi didn't seem to be affected), and eventually the line output came back.
Figured it was just a fluke. If you have other audio outputs you can try, I'd test through those, confirming software settings (I noticed especially Firefox got muted a lot).
It was intermittent enough in my case that it was hard to say if it was an OS problem or just bad software.
