I'm on my Snow Leopard MBP right now.

QuickTime X does not even support WAV files. That's how limited it is. If you play a WAV file in Safari, it invokes the old 32-bit QuickTime 7 Plugin.

However, if you play an AU file, it invokes the new QuickTime X plugin, which then invokes the old 32-bit QuickTime 7 helper process.

An older-style MPEG-4 Part 2 movie is supported natively. MP3 also plays natively. A MOV file containing uncompressed audio plays natively.