If you are the GM of the game, go to the Jukebox tab (4th tab, has the symbol of a musical note) and scroll down the list until you find the entry which is playing. You can remove music via the 'remove' link that appears when you mouseover the specific musical entry. In this image, you can see that my Rain Loop is playing, because it has a 'pause' rather than a 'play' graphic. If this doesn't help, report back and we'll get into troubleshooting.