I had the same problem as the poster above. This problem happens in Firefox on Linux if you have a flash component in one of the browsers other tabs that causes firefox to grab control of the soundcard, so that Java is unable to get exclusive access to the card when it needs it (Java does not go via firefox).

Essentially the problem is that niether firefox nor Java connect via ALSA which allows all applications to connect at the same time, but try to take control of the raw sound device themselves. This is what I see when I run the lsof command to see which applications are using the sound devices:

root@dominic-laptop:/etc# lsof | grep snd
mixer_app  5578    dominic   19u      CHR     116,10                14218 /dev/snd/controlC0
totem     30649    dominic   17u      CHR     116,10                14218 /dev/snd/controlC0
firefox-b 30797    dominic  mem       CHR      116,8                14183 /dev/snd/pcmC0D0p
firefox-b 30797    dominic   44r      CHR      116,2                13897 /dev/snd/timer
firefox-b 30797    dominic   49u      CHR      116,8                14183 /dev/snd/pcmC0D0p

As you can see, whereas totem and mixer_app connect via ALSA (controlCO) at the same time, firefox connects directly to the raw sound device (pcm...) and so prevents Java. This problem goes away by closing the tabs that have sound producing flash controls.