As a cross-platform Windows and Mac user (I really have to be as a
writer), I can tell you I really miss Mac OS X's brilliant Core Audio
when I'm on Windows. And looking at Apple's updated Core Audio page for Mac OS X Tiger, Core Audio is only getting better:

• Core Audio file format allows 64-bit files (bigger than 4GB), supporting any QuickTime 7 encoding (uncompressed PCM, MP3, AAC, whatever you want)
• MIDI/Audio Device Aggregation takes the easy
  multi-interface support in X and ratchets it up a notch, with the
  ability to use multiple devices as though they're one device, assign
  different channels to different apps, etc. This should make MIDI and audio routing easier than ever.

(read more for upgrade advice)