My first guess is that you are a little short on system resources. Second guess: you are using MP3 files as an audio source instead of standard 48Khz 16-bit PCM WAV or AIFF files as sources. My third guess is that you have not selected the right audio options in the Preview Panel and/or your preferences file.
After Effects is not a video editor. Everything must be rendered to available RAM before it will preview correctly.
System info, AE version, OS version, and workflow details help us answer questions. Without something to go on, you might as well ask "Why is watermelon?" There is no answer to that question.
Audio preview is often about RAM availability and how quickly your computer can access the cache. I find that there's only so much AE can do, and sometimes, you jsut have to audio preview and use the * on the keypad to set layer markers and time your animations to that.