Can I make a song with vocals into a Karaoke version?
Audacity already has some plugins that you can use to remove the voices from a file, but I don't think it can export to a known format used in Karaoke mixing. For example, the Karaoke format uses MP3+G, but that isn't easy to achieve.
For that operation, you will have to pick an application from the following result list: http://software.informer.com/search/karaoke+creator
