In ALL legal questions... think like the Musicians and Song writers. If they have been paid for the song, then it is in legal use.
If a customer brings his own Audio CD to you that he has bought, you can remove the vocals, as best you can (be sure not to PROMISE you can), make a new Audio CD with the reduced/removed vocal song, and charge for your time. If he brings a copied disc, his word is all you can go on and you are clear if he lied.
However, if another person wanted the same song, and she had NOT already purchased the Audio CD with that song, then it is illegal. To be legal, have her go out and buy the song, then give her a copy of the vocal reduce/removed version you already processed... and charge her for the time also.
