sound/music probs while playingFollow

Hello everybody,

i have a problem with my sound while i play FFXI. It seems only affect the music of FFXI and not the effects. Heres the prob:
The music appear to be "lag", like a platter which is broken, even when theres no lag in the game tough. The problem isn't all the time but it appear often. Now i don't know if it's the fault of my maybe to slow machine (AMD2000+ 256MB), wrong directx version (latest DirectX 9.0c) or something else.