Realize that you could create a noise dependency... but classical music is great. Slow moving pieces like Vivaldi's "Winter" or Rachmaninov's Piano Concerto #2 in C Minor.
I used to play classical music for my son when he slept because many doctors told me it would soothe him!! It is very relaxing to the ear and it calmed me as well! I would recommend that you play something like that because it stimulates the brain at the same time it calms!
Uh, how about nothing? Billions of kids don't have background music while they sleep, why do yours? If you feel you have to play something, do something classical like Mozart of Beethoven.
Children music like Care Bears are all ways secure pickup. Modern TV series
other hand is good yourself check what its about. Rock, Hip Hop, Classic good choices.