This is probably just me, but whenever I start a book at 1x, it sounds okay, if a bit slow. Then I increase speed to ~1.5x for a while, and, when I listen on 1x again, it seems unnaturally slow, to the point where I wonder if audiobook producers slow the recording down a bit on purpose, just to make it clearer.

However, the fact that I usually notice it after listening on 1.5x makes me think that it's really an illusion. Does anyone have any "insider" info on this?