Is there a karaoke trainer for OS X?
You could give UltraStar Deluxe a try:


  A free and open source karaoke game inspired by the Singstar™ game available on the Playstation®. It allows up to six players to sing along with music using microphones in order to score points, depending on the pitch of the voice and the rhythm of singing. 


The content is somewhat limited though.

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