90% of artists music isn't owned by them, meaning you don't have to ask them, you have to ask the owners. When a track is signed to a label, it is the property of the label to do with what they want. For your purposes, this works in your favour, because most big labels have H&R departments that deal with this sort of thing daily. But it all comes down to the track and what publishing rights are in place for each tune. It's a WHALE of a thing to deal with, and if you were signed you can usually just ask your label rep to get clearances for you..