I was just thinking about which games for the 2600 have constant music in the background as you play. Not just intro or event tunes, but actual repeating BGM. It's a stretch to include things like Space Invaders which really just have a sound effect relative to the aliens movement, but I'll accept those types of games so long as the sound is constant and rhythmic.

I'm sure that here are a lot; here are a few I can think of:

Demon Attack
Halloween
Smurf: Recue in Gargamel's Castle
Space Invaders
Strawberry Shortcake