You have to add some music and sound effects to the game... They were so iconic in Shadow of the Beast and the Atari 2600 is capable of pretty good audio. I hope you keep working on the game, but it desperately needs background music and sound effects.
My experience tells me sound effects may be possible but you really, really don't want to attempt a soundtrack. Everything on the Atari 2600 is a tradeoff due to limited ROM, RAM and CPU time. In other words: the more features you want to add the less existing effects are possible. You might have to re-write the engine to fit just one more feature.