Michelle,
This thew me for a loop, too, when other compositors seem to play/scratch audio as you preview the timeline..
To do it in AE: you can hold down CTRL (Option on a MAC I think) and scrub the Current Time Indicator to hear a scratch track..
Or, you can hit ‘.’ (decimal) on the numpad to play the audio, but the video doesn’t update..
Or, you can hit ‘0’ (zero) on the numpad and it caches the work area and plays back the audio and video (but this takes a few seconds)..
-Matthew