I dont have a solution for you, but some tips..
- Run xmms from a terminal to see any possible error messages (copypaste here please)
- If it freezes, kill its freezy bony ass.. (; No need for a reboot (if I understood correctly you referring to restarting thw whole system) If you arnt familiar with killing processes,
clicky
- and as last, you have libmpeg123 installed, right?