If you think it's the file type that's the culprit, can you indulge us by divulging the identity of the file type?
It is a MP4 File.
...and an MP4 file can contain multiplexed, or muxed, audio, I believe: the audio data is embedded in the video, and not in a separate track. That's why you can't hear it or click on the little speaker icon.
An MP4 is a media container. It can contain video encoded in different ways, one of which can be mpeg, and mpeg can be muxed.
The fix: export the clip as a quicktime movie with an audio track in the PNG codec, which is lossless. You can use anything BUT AE to do it: Adobe Media Encoder, MPEG Streamclip (free download), one of Telestream's fine products, Quicktime Pro, etc.
So you are saying I have to convert all my video files? Well what to?
Hmmmm. I'm pretty sure I wrote, "The fix: export the clip as a quicktime movie with an audio track in the PNG codec, which is lossless."
You mention "all my video files." Too bad there are so many.
I wouldn't want to do all that conversion. First, I'd want to make sure the the assumptions of some guy in the AE forum are correct.
If I'm you, I'm checking just what kind of video your camera shoots.
I'm highlighting the file AE's project window for info on it.
I'm double-checking the camera manual.
I'm looking in the editing application I use for info on it.
And I'm reporting back in.
I Hope that you have tried the Ram Preview.
This is the only way you would get the audio while playing back the comp.
Also, are you talking about the Render Queue and after you outputted the comp you are not getting any sound in it?
If you are talking about the render queue, make sure that audio option is also checked before clicking on render.
Have you simply considered getting rid of any CoDec packs on your system (if you installed any) and reinstalling AE and Quicktime?