what you should do instead of using songs currently under copyright, create midi tracks of public domain songs, it will still show your skill at midi sequencing. You can then create your own website and put the link on your soundcloud account for the tracks you create. Karoke is no different from a full version song, it will still be copyright infringement to use the melody of a song; so unless you purchase the license you can not sell them. You could however offer to turn peoples original songs to a backing track and still make money if that is your goal, and by showing your backing track creation skills you may grow a client pool. People do post cover songs, a site will generally not buck unless you are selling the copyrighted tracks, unless someone associated with the artist protest. Then you will be given a warning to remove the track immediately or risk prosecution.

My Tunes

Psalm 57:7 My heart, O God, is steadfast, my heart is steadfast; I will sing and make music.