WebmasterWorld administrator, Tedster, started a thread at WebmasterWorld suggesting that Google may begin indexing audio files in the near future. Tedster linked to the recent news on Google announcing that they are "automatically" transcribing YouTube videos "from speech to text" and then indexing that content in a searchable format.
Personally, I think if they got that technology down, it would simply rock. But it would make much of the audio files, MP3s, videos and multimedia content all searchable.
Many operating systems have speech to text technology. They make software that tries to transcribe your voice as you speak to it. They make technology, OCR, that reads text as an image and translates it to machine readable text. None of this technology is perfect, or even close to it. Google has dabbled in all of this in the past and I am sure they continue to experiment with this now and in the future.
So keep watching the accuracy of search results such as filetype:mp3.
Forum discussion at WebmasterWorld.