12.3 no sound you tube in firefox

[michael norman]

Running 12.3 KDE4 4.10.5

I have Xonar STX soundcard, everything works (VLC, Clementine even
Netflix using pipelight) for sound.

I have been running Linux Mint mainly for a while as that was the only
way I could get Netflix to work. Anyway having been able to get
pipelight to work in 12.3 have reverted to 12.3 for my main use.

Today I tried to run youtube videos from Firefox and whatever I do I
don't get any sound. System sounds and all other multimedia progs work
for sound.

I have Mint in a vm and using the same hardware youttube videos work.

Welcome suggestions where to look for the problem please ?

Mike