If you hear a 100 Hz tone in one ear, and a 110 Hz tone in the other ear, your brain will automatically synthesize a third tone at 10 Hz, which is the natural frequency of the Alpha (dream) state. Of course it can be boring to simply listen to a tone, so the creators of these audio files typically blend in some other pleasing, relaxing sounds.... Maybe jungle sounds, maybe surf or running water...whatever. They can also lay on a dialogue track to produce a "guided" experience. None of this interferes with the binaural beats, which are just the phase difference between the left and right channels.