I was using Firefox 128.0 on KDE 6.x to work in a web app that includes audio. Suddenly, the audio stopped working in Firefox. I tried a YouTube video, and had no sound from it, either, despite the speaker being un-muted. Audio was still working in other apps (Quod Libet) and the same sites that had no audio in Firefox worked fine in Chromium.
I tried closing Firefox and running it in safe mode, but neither helped. I tried another PC running EndeavourOS/KDE that hasn’t been updated for a week or so, and audio worked fine in Firefox on it, so I was certain the issue was with the problem machine.
I finally found the problem and the solution.
When I ran pavucontrol from Konsole with Firefox silently playing a YouTube video:
pavucontrol
I saw on the Playback tab in pavucontrol that Firefox was using the correct audio output device, but the “mute” button was pushed. I un-selected “mute” in pavucontrol, and the audio in Firefox began working again.
I have no idea how the mute button in pavucontrol got pushed, but I thought I’d post this in case it helps someone else avoid all the troubleshooting I did if the same happens to them.