In all the New Super Mario Brothers games, the music in all the levels, have a voice saying "Bahh" in them over certain parts of the level music.
The enemies as well as power up items will jump and sometimes do a little dance depending on how many "Bahh's" were said at each time (enemies only, and I think only in Super Mario Brothers 2)
This happens in all the games, and all the levels, each enemy will do something different each time.
This can be seen by the goomba's, 49 seconds in and again by the Goomba and Fire Flower at 1.39.