[ScottBurson]

> sound is a waveform so even the loudest notes will oscillate around zero

Sure. When I refer to "peaks and valleys" I'm talking about the envelope of the waveform. I think that's standard usage in audio engineering.

I think you should download Audacity or equivalent and look at some waveforms of both uncompressed and compressed music. I think you'll see that there are loudness variations on a much shorter timescale than you expect, and that compression reduces these considerably.