A thread on WebmasterWorld (via SEO Roundtable) suggests that Google is close to adding a new type of content into their indexing mix. To date, in addition to typical web pages, they index PDF files, Word docs, Excel, PPT, RTF, txt, and swf and a few other formats found here.
Adding MP3 into the mix would be incredibly cool since a ton of very interesting content found on the web today is hidden inside audio podcasts.
Many podcasters have been going through the work of transcribing their podcast productions in order to generate easily indexable text transcripts. Should Google manage to auto-transcribe the contents of MP3’s for indexing, this may become slightly less important.
However, my initial take is that transcription would still be valuable since Google will still have an easier time indexing text over taking their best guesses at what was said in audio files. Hard of hearing and non-native speakers are also big fans of transcripts since it allows them to consume or read along with audio content that would otherwise be inaccessible.