Is it just music or do all sounds play like "chipmunks" (though you fail to specify which chipmunk it sounds like, Alvin,Simon or Theodore, it may make a difference;))
Sorry seriously, some sound cards do come with software that enables you to alter the sound by putting "silly" digital effects on, these include things like underwater, cave, and probably the silliest sounding one is a chipmunk setting (i am being serious!)
These settings would, in most cases, be accessed by a small icon down by the clock, if not through the control panel, check that someone hasn't turned on the chipmunk setting, probably as a wind up.