Where does the file come from? What are the exact specs? What system are you on?
It could be that your videlo clip has no separate sound track, and essential in AE. MPEG files can have multiplexed, or muxed, audio: the audio is embedded in the video track.
No separate audio track, no audio in AE. If you need audio, you can export it in a lossless codec with an audio track. Use a different application than AE, of course.
Dave's suggestion might solve your issue. However, these are the first things to check when previewing audio in After Effects:
A) Make sure the audio switch is on for the clip.
B) Open the waveforms and make sure After Effects is reading the audio.
C) In the Preview panel make sure the audio switch is not set to mute.
D) Use the RAM Preview button to play back the clip with audio. After Effects won't play back audio without the use of the RAM Preview button.