Which mime type should I use for mp3

Your best bet would be using the RFC defined mime-type audio/mpeg.

I had a problem with mime types and where making tests for few file types. It looks like each browser sends it's variation of a mime type for a specific file. I was trying to upload mp3 and zip files with open source php class, that what I have found:

• Firefox (mp3): audio/mpeg
• Firefox (zip): application/zip
• Chrome (mp3): audio/mp3
• Chrome (zip): application/octet-stream
• Opera (mp3): audio/mp3
• Opera (zip): application/octet-stream
• IE (mp3): audio/mpeg
• IE (zip): application/x-zip-compressed

So if you need several file types to upload, you better make some tests so that every browser could upload a file and pass mime type check.