After Effects has to load everything into RAM in order to play it. Often this doesn't happen in real time, depending on your hardware and the complexity of your project, and After Effects plays things slow as it struggles to keep up. (Personally, I find the audio pitch change function in AE previews useless. I'd rather AE just didn't play the audio if it can't do it in real time. You can set this option in Preferences)
Things to solve the problem: wait for AE to build the preview, reduce your preview resolution, shorten the area being previewed.