I get this problem sometimes, and I think it has something to do with playing music at the same time as using RM2K. If I do something strange with WMP then I end up not being able to hear the music on RM2K. It usually fixes itself if I close all the other programs I am using, re-open them all (with RM2K last) and carry on from there...
If that doesn't work, then try rebooting your computer whenever this happens to you. But it is something to do with using other media players at the same time as RM2K that seems to activate/deactiate the music randomly.