You can use the "Upload file" link from the toolbar at the left to upload an Mp3 file to embed in a page, but you can also embed an external Mp3 file you have set up on your own site. Just use the following code:
<mp3>uploaded filename.mp3 or external URL</mp3>
or
<mp3>uploaded filename.mp3 or external URL|download</mp3>
The "download" parameter adds an icon with link to download the mp3