I’ve had this exact problem before.
Audio being routed to a dummy output device.
Offhand, I can’t remember how I fixed it, because it was a while ago.
I only had the problem after using an external, USB audio interface, to record a demo of a song.
The day afterwards, I restarted my laptop without the audio interface plugged in and discovered that I had no sound.
Looking in ALSA and pulseaudio - sound was being routed through a dummy device and I had no way of selecting the onboard sound card.
However, I solved the issue myself and I’m sure I posted something about it here, when somebody else was having problems with their sound.
If I find it, I’ll edit this post with a link to it.
Edit:
This was the solution when I had the dummy interface problem after using HDMI output.
Here's a heads up about an unusual audio related problem I finally fixed on my laptop the other day (running Debian Testing). A couple of weeks ago I had my laptop plugged into one of our TV's via the HDMI port and had all of my audio routed through the TV so my wife and I could watch some...
www.linux.org
I can’t find the solution I posted about the dummy interface used after using an external USB interface though... That may have been on one of the posts that was lost from one of the older iterations of Linux.org. Or it may be that I posted that one in a different website......