Problem
Sound doesn't work (for Flash plugin) in Mozilla Firefox, but it does sometimes work for other applications.

Solution
First, stop any application that is also using sound (such as xmms), restart Firefox, and try again. If that didn't help, edit /etc/mozilla-firefox/mozilla-firefoxrc and change

FIREFOX_DSP="auto"

to

FIREFOX_DSP="none"

or

FIREFOX_DSP="arts"