I do know that the use of PDF files can rank well within Google and I recently read that one of their patents was for detecting & ranking different files (for the life of me I cant the link to the story).

For now I'll guess they do detect different media type, I know that Yahoo does it with videos and one example of Yahoo and Google would be their image search.

Personally I would use the keyword-keyword.mp3 layout because not only does it look clean for the user but I would suspect Google is going to get smarter on detecting files type (or at least try). This would save you in the future of having to rename any of your files.

Thats just my personal opinion though