From top level (My Songs), lower left corner icon, you can select "Sharing" to either iTunes or Mail. If selecting iTunes, you can either upload the song in iTunes or GarageBand format.
In iTunes, sync device, then select App, scroll down to File Sharing, select GarageBand, and you'll find the uploaded file (either format) that you can then save locally to play or edit.
Note you can only go one way with GarageBand format (from iOS to MacOSX), but you can use the same file sharing approach to download audio format files for editing in GarageBand for iOS by dropping the audio files in the File Sharing area.