OK, so you're already using the nice ALSA sound system on your Linux install, but when you're playing music etc you can't hear the sound of those YouTube movies? Edit /etc/firefox/firefoxrc and add/edit the FIREFOX_DSP setting to read "FIREFOX_DSP="aoss". Of course, you will need ALSA's OSS support too, so install alsa-oss and restart Firefox.

Enjoy!