I've been using Firefox since a long time but as I remember it only appens since I've updated Flash to 11.3 ( I currently have 11.5.502.146 installed.
So when I go on youtube for example to see some videos it works find but when I pause the video, wait a few seconds and then want to continue it the sound isn't there anymore.
I wonder why it only happens when I do such things.
( I checked the sound level from Windows but it wasn't muted . )
EDIT : I don't know if it helps or not but Flash works fine in Opera 12.11 x64 .