First, does sound work on the rest for the rest of the software? Dumb question, I know, but had to ask.
I have not had any issues with this on my 10.1 box, but you may want to try starting the browser on the command line and look for stderr output (error messages printed to the screen). Or you can try starting the browser under artsd ( KDE uses arts for sounds ). It would look like this:
user@system$ artsdsp mozilla
Hope this helps.