Running Debian sid and it mostly works (still struggling with Nvidia drivers for video). Last night I was watching a video and everything was fine, sound worked. Son was in room "asleep". I left to use bathroom and then he came looking for me. I don't know what happened while I was in bathroom but now I hear nothing out of speakers. If I reboot into windows sound works so it's not a physical issue. Reboot back into Linux no sound. Boot off flash drive Linux, no sound but I think that was always true and was a alsamixer issue on flash drive Linux setups. Volume on YouTube turned to 100, verified not muted, volume in KDE at 100 and not muted. Physical volume on speakers at 100, volume knob on keyboard at 100 and mute not engaged. All volume sliders in alsamixer maxed out. Simple setup, 2.1 speaker system plugged into sound output on motherboard no mixer or fancy equipment.
Son is non-verbal and severally autistic so I can't ask him. Whatever happened was almost definitely an accident but I can't figure out what happened. I see like a lighter bar moving on the KDE volume slider which seems to match expected video audio so I believe KDE is receiving the signal to play sound but nothing comes out of speakers. He struggles to feed himself soup or cereal so it's not like he found some super secret command and is playing joke. Something seems to be accidently hit and is blocking sound... Maybe a mute hot key not synced to main audio controls so it's not obvious that it's active???
Edit... This is solved. KDE mixer and alsamixer made no change but changing the output in pavucontrol fixed it. I have no idea why or how. Thanks for everyone who recommended it.
If you go into KDE's settings > Sound section, what devices does it show, and what type of output (like "Digital Stereo" or "Pro Audio") are they set to?
Once in a blue moon, my audio output will get mysteriously changed to a format that does nothing, or the audio controller will go *poof* and I need to manually re-enable it. It's rare, and I've never been able to pin down the exact cause, but it seems more likely after I've been connecting and disconnecting alternative audio devices, like my PS4 game controller (which also reads as an audio device due to its headset port), USB microphone (same deal), or USB headphone amp.
After all these years it's still a pain point.
It's "digital stereo" currently, I tried all the others no change.