It seems pretty obvious. Everyone has a smartphone with music on it. Not everyone has movies on their smartphone or want to watch a movie on a small device. Thus Delta providing movies on a larger format display is extremely more valuable than them focusing on providing music that 99% of passengers will never use. And the ones that would have used it probably don't end up with absolutely nothing to do - they just listen to whatever they already have. Doesn't seem like a business needle-mover in the slightest, and I doubt that royalties are that much of an issue, it's just the effort isn't worth it at all.