AFAIK, Google accepts sitemaps which are about ordinary URLs (text/plain), images (image/png, image/gif, image/jpg, etc.), and videos (video/avi, video/mpeg, etc.).
My question is, how many Internet Media Types (formerly known as MIME Types) can have sitemaps. For example, if I'm a researcher, can I create a sitemap for my PDF files (application/pdf), or can a compose create a sitemap for his/her compositions (audio/mpeg3)?
To the last bit about composers, this could be done by extending the sitemap format with an audio namespace. (This is basically what's done for the video sitemaps, eg. <video:description>.)
But that only covers the technical possibility.
The actual practical use of such a sitemap would be limited at best right now, as Google isn't doing anything for audio that's analogous to their image or video search functions, which may use the sitemap data to let you filter video search results by length, for example. I don't think any major engine is currently.
If Facebook is a factor for you, apparently they suggest cramming audio into their video OpenGraph tags, but the Google employee in that thread is definitely suggesting that wouldn't be too great an idea for sitemaps. The person who opened the thread does bring up using HTML schemas for it, though, which you might investigate.
Google's advanced search shows that Google supports these formats:
So if your content in these formats and want them index they should be in your sitemap.
