I’m not home so I can’t experiment with this at the moment, but it seems to me you could do this in Garage Band.
Just import the karaoke backing tracks into a file…you could even just use one file to hold all of them, then selectively mute the ones you don’t want to play back — I think you could even solo the chosen track, and that would mute all the other ones.
Then set up a vocal track (with appropriate effects added in) and solo it too. Make sure that monitoring is enabled. You should hear the track playing back plus the vocal one. (Just be careful about speaker placement to avoid feedback.) I don’t think you’d have to actually record to hear it, though you could.
I could be talking through my hat about all of this, but in theory I think it would work. Maybe a Garage Band expert could chime in about the possibilities.
OK. Sorry, my ignorance. I’ve done karaoke exactly twice in my life, and I disdained the lyric screen as they were songs I knew well. I’m not aware of how a karaoke file is provided. Is it a proprietary format or just some sort of word processing file?
I’m pretty sure singsnap.com supports Macs. It’s a streaming karaoke site with a fair number of free tracks, though most require a paid subscription. If you can find enough free tracks there that you like, it might work for what you want.
Otherwise, if you have your own files, you could play the lyric tracks in VLC and the audio in something else and just start them both at the same time.