Have you always had this problem? Or only since using MAME32? I seem to remember having a similar problem when I used MAME32 - I seem to remember the frameskip option was to blame. I think setting it to 'Auto' rather than '0' may help (?).
Also which games? Some games just seem to sound a bit 'crackly' to me (eg some revisions of MK2).
I doubt very much it's your soundcard
