I spent an hour or so listening to various bits of music -- all ogg format -- specifically to catch these pitch problems. Found nothing, no pitch changes. The worst is a short click or very short pause when volume is changed with the physical button. Working with the Reader app on the eInk display or the Kindle app on the LCD screen (which scrolls pages horizontally) paging as fast as I could tap the screen/buttons didn't introduce any variation in the audio.

This is interesting, because I had been under the impression ogg playback required more resources than mp3.