Firefox hogging up my sound card (Slackware 12.2)
 

If I play a youtube video and stop it, then try to start xmms and play an mp3 it says that another program is blocking the soundcard.

Is there a fix for this?

Hi;
I use ALSA output plugin for xmms and have no problem.
Try to change default output plugin.
(Preferences > Audio I/O Plugins > Output Plugin)