I have the same problem here. Does anyone know how to fix it?
Audio broken on Firefox
It's been various types of glitching for me too for as long as I've used Firefox (this year). It's very frustrating and weird sounding and inconsistent. Running Mac OS 10.14.
Just opening dev tools fixed it for me. Based on the sound I'm guessing there might some bug where it's playing multiple versions of the same sound at once. Weird clipping/phasing.
You could try something like "strace -o ~.logfile firefox" to run the browser, and submit the debug file (which may well be very big) to Firefox devs.
This is likely due to https://bugzilla.mozilla.org/show_bug.cgi?id=1730499, which was fixed in 92.0.1 by reverting a change in 92. This may have caused https://bugzilla.mozilla.org/show_bug.cgi?id=1687070 to come back, and cause some very short sounds to not play properly in Linux. Switching to another slightly longer sound may well fix it. It doesn't happen to everyone -- I for example have never seen it on Linux.
The full fix for this is in 94, which is going into beta shortly and will be out in a month and a bit.
Audio is distorted on Firefox 92.0 (64bit for Ubuntu) for piece moves in live chess. You can hear a very faint sound of the piece move but it's certainly not functioning correctly. No issues on chrome.