Having music playing in your mind happens to most or all humans. In my case, sometimes I will pick something to play in my brain, but most of the time it plays random snatches of tunes. Much of the time my brain will start playing something based on association. A word or phrase that I am reading, or hear on the radio will immediately trigger a tune that is associated to it in some way. I don't let it get to me. If a tune gets "stuck" in my mind, and starts to annoy me, then I just hit my mental "jukebox" and play a different mental tune. There are a lot of tunes to pick from, so have fun with it. I like patriotic tunes, some classical, some early rock, some show tunes, and some folk and popular tunes. There are also some TV show and movie themes I like. I also like some Irish music, Polka, and other ethnic music. One of my favorite CDs is the music from "Henry the V". My sister gave me the cassette for that years ago, and kept the CD. By the time I wore out the cassette, she had lost interest in that music, so she sent me the CD.
Music can be a great mood booster, as can exercise, and humor. Because of health problems my ability to exercise is very limited, but I can listen to all the music I want. I also listen to funny old radio shows, and sometimes watch old comedy movies and TV shows that I have on DVDs, so I get a good dose of humor, too.
Just have fun with your mental music show. After all, it's your head, so you can always change what's playing.
Remember, we on the spectrum are all:
A Different Drummer
If a man does not keep pace with his companions,
Perhaps it is because he hears a different drummer.
Let him step to the music which he hears,
However measured or far away.
--Henry David Thoreau