after effects has to render everything...so you never get playback at 100% quality and 100% speed. you have to use the rampreview option to render frames out. as default after effects doesn't bother with the audio....but i believe there is a way to turn it on?
In the RAM Preview window there is a button for audio.
AE was not built for realtime previews so as stated above, u need to use the work area bar to RAM preview a selected section of your timeline. To increase speed you can set the preview mode to 1/2, 1/3, 1/4 resolution, or in the RAM Preview window you can get it to only render every 2nd, 3rd etc frames.