AE - Imported mp4 seems to be missing audio
If you just play the comp it won't play audio. You need to do a RAM preview. Hit 0 (zero) on the numeric keypad or Ctrl+0 if you're on a laptop. 

. (period) on the numeric keypad (or for laptop ctrl+.) plays just the audio from wherever the playhead is.

If your footage does have audio, you will see the waveform in the preview thumbnail in the project window, and the item description will tell you the audio format, as below (in this case 44.1kHz, 16 bit unsigned pcm, stereo audio)



If the clip has audio and you bring it into a compositon you should see a little speaker icon on the layer with the audio, next to the eye icon, on the left.