Ah, thinking slow today. Yes, I should have suggested trying with other browsers to see if FF is the problem.
It could be a Flash problem but it could also be something wrong with speakers/sound card/sound drivers. I'm pretty sure you can tell the difference between "speakers don't work properly" and "specific sounds don't work properly" but it is worth checking just in case[footnote]Once, I spent half an hour fixing my speakers when I played an MP3 that only had sound for the right channel...[/footnote]. If it's still only browsers, see if Baldur's gate website [http://www.baldursgate.com/] is still having problems (it's the only one I can think of right now that doesn't use Flash to play the sound). If it's still bad...I have no idea, I'd say it's something wrong with your machine. If not, it's probably Flash, try upgrading. Although, it is possible that it was an update that broke it, in which case...I don't know again.