I had the same question about PCI sound cards. I know with Zorro/ZII sound cards like the Delfina series you can use the card to decode mpeg audio instead of the CPU. There are plugins available. In programs like frogger (video player) you can specify Deflina, AHI, etc. There are also mp3 decoder boxes for the parallel port but they are probably harder to find.
Elbox were supposed to release drivers for PCI DVD/MPEG decoder cards as well.
