Putting music on your Xanga site is very easy. First you must find your song. Right click the link to the song and click "copy shortcut." Go into your Xanga "Look and Feel" section and scroll to the bottom of the page. There should be a box that says "Music URL" above it. Paste the link from your song into this box. If you want the song to play over and over while a visitor is at your site, check the little box below it that says "loop." If you want the song to play once and stop, do not check the box.
This is a great place to get MP3s. They have any song you could ever want. A lot of their songs are about 30 seconds long, so if you want a full-length one, look carefully.
I really have not found a better place for MIDIS than than here. They have quite a few songs, but most of the ones they have are from really famous bands and singers.