I had the same problem, I just reinstalled the whole package and it was woking again.
(are we talking about a apple or PC)
What kind of audio and video files are in this imported project? AE doesn't do well with some MP3's and, if your footage uses any kind of interframe compression, AE can freak.
I quote my friend Dave LaRonde from CreativeCOW.net:
"If the footage you imported into AE is any kind of the following -- footage in an HDV acquisition codec, MPEG1, MPEG2, mp4, m2t, H.261 or H.264 -- you need to convert it to a different codec.
These kinds of footage use temporal, or interframe compression. They have keyframes at regular intervals, containing complete frame information. However, the frames in between do NOT have complete information. Interframe codecs toss out duplicated information.
In order to maintain peak rendering efficiency, AE needs complete information for each and every frame. But because these kinds of footage contain only partial information, AE freaks out, resulting in a wide variety of problems."
If that's not it, how are you trying to play back the audio? Have you ever gotten audio to play back in AE? Have you checked AE's audio hardware preferences and audio output mapping preferences?
Have you turned your speakers on?
Have you tried using the zero key on the numeric pad to preview with audio? Cin