Only songs you hold the rights to. So something you wrote yourself or with a group. You can do an original performance or arrangement of someone elses songs though but even then if you were to try and sell it you need to pay licence fees, it gets a little strange here on Steemit because it works in a different way where you are not selling anything but being rewarded by people that like your content, so to look at a release here as a commercial transaction would be kinda incorrect. Be aware that everything you share is freely downloadable though. For a small artist like me it's great, someone actually listening to my music is just an amazing thing for me :)