It seems most of the problems relating to sound are caused by 'bad' or incompatible codecs installed, usually through a pack or a third-party player, with that codec taking over the playing of MP3 through DirectSound. This is probably also why playing WAVs instead works.
You could try unregistering this mmswitch.ax file by typing the following in Start Senu > Run:
regsvr32 /u mmswitch.ax
Also here's what a quick google on this came up with:
http://www.inmatrix.com/zplayer/faq/faq_entry0027.shtml
http://forums.designtechnica.com/arc...hp/t-1399.html
http://forums.divx.com/groupee/forum...51/m/622103981
I've been playing around with codecs and A/V splitter files lately so I know this can sometimes be a real pain to fix
Hope this helps,
Stian