I read a lot of other forums concerning this idea, and I have tried most of the suggestions. Surprisingly, what I found is that the video files the software doesn't play are from my camera while the video files it does play, is downloaded from the internet. By playing, I mean playing the audio with the video. When I tried to look at the audio wave, it was missing for the files that had no sound. The other MPEG-4 files that did have the sound, had audio waves. For example, the file M4H01572 was the one that had no audio playing while the other file has almost the same properties but it was still playing audio when rendered.
There are about a zillion ways to encode audio for MPEG files. This particular format may not decode well in AE. I'd try Adobe Premiere or the Adobe media encoder. If they can playback the audio I would transcode your files to a suitable production codec using the standard 40khz 16bit PCM uncompressed audio. Something like Quicktime JPEG or PNG file would do. There are presets in the Adobe Media Encoder that should work for you. Just match the frame rate and frame size.
Thank you! I was able to fix it.