It works with free and pro accounts.
You won't see a mp3-player, the music will automatically start to play.
Follow these steps:
1) First of all upload your mp3-file
Go to design (on top of the page) > Edit HTML/CSS (bottom left) > upload your .mp3 file under files (left side).
Get the link: http://"YOUR NAME.weebly.com/files/theme/FILENAME.mp3
2) Go to the page where you want the music > drag a "Embed code" element on the page (bottom of the page) > paste the following code in it
Paste your adapted link in the code, and the music will start to play.
( If you don't want the music to loop just change: auto loop="false")
Example on dddproduktionen site: http://dddproduktionen.weebly.com/backgroundmusic.html
Click here for the video tutorial: http://www.youtube.com/watch?v=s67MOb_Gchc