You don't have to set mime type in C2. Audio is handled by the audio plugin and will choose, depending on the platform the project is running on, what format to play and how to play it.
Just add your music to your project, and play it using the Audio plugin.
Look through the manual entry for further explanations on how to play audio, apply effects, loop, ... It's pretty easy to use, and you can thank a lot Ashley on this one, as the Audio in HTML5 isn't as simple as it looks like if you want it to run fine everywhere.