Which one do I use for a song that I'm listening in a loop?
I asked this on Quora but didn't get a satisfactory clear answer that would justify the difference.
I don't think there is /much/ of a difference. However, "relentless", I believe, means "without end", while "unrelenting" includes the idea of "yet" or "so far".
I think the more literally accurate sentence for your situation might be:
"I'm listening to X unrelentingly".
Other choices might be:
I'm listening to X... ...continually ...ad naseum ...repeatedly ...over and over ...continuously, in a loop from where they say "aaa" to where they do "bbb" ...forever more, until I die or my Mom pulls the plug
Do you mean music going around and around in your head that just won't stop? A common term is 'earworm', explained as: 'a piece of music that sticks in one's mind, even when it is not being played."
(happens to me all the time, both with tv ads and music I play)