you could do a mix of a popular tune if you did your own tracking of the instruments of singers and if you did not sell it. So if you wanted to give a remix of a popular tune away, while using your own singers, then you should be fine.
That’s what I’d like to do! Now I feel less overwhelmed about the whole copyright thing…I’ll just make it so that people can download the tracks for free. Besides, the end result (Salvation) is free!