I don't have a song running 24x7, but a good portion of my time — unless I have my earbuds in, to make sure I'm listening to a specific music for, say, keeping the pace up on our treadmill or walking outside.

Variation — if I have an old familiar tune running in my head, it will sometimes switch to the next song on one of my old mixtapes (which are drilled into my head from long drives — the tapes themselves are long gone). When I am listening to a streaming song that was on an old mixtape and the next song is NOT the "right" one (which it never is, because what are the odds?) I have a momentary "wait, that's wrong!" reaction.

Mostly I like having my own personal elevator music running.