Connect a once to a sound. Put a mp3 on the sound and there you go.
This is already posted:
http://forum.flowlab.io/discussion/82/here-is-how-to-add-music-to-your-game#Item_8
the only prob is that do we put in the site we got it from? or what
Use kiwi6 and copy the direct .mp3 line from the Share button once you upload your song.