I don't know if it's in every level, but I was playing castle 1-4 and noticed a minor bug. First, I had the volume turned all the way down. I died, then restarted the level. I turned the volume all the way back up, and all of the sound effects worked except for the music. However, I died again and the music came back. So really, it only happens when you die and then unmute the game. Again, it's not that big of a deal, but a bug nonetheless.
Thank you for constantly improving an already awesome game!