It depends on what model of camcorder the video came from and what format and resolution it is.
But it sounds as though you merely didn't give the program time to fully conform the audio before you started editing.
Clear the video clips from your project and then re-import them. Wait for the program to fully conform the video and audio (as indicated by the messages along the bottom of the program) before you add the clips to your timeline.
When previewing, the clip plays without the sound too. All the clips are from the same source, a Samsung camera, in mp4 format. If I clear the files from the project and re-import, the same thing happens. If it was a conforming issue the first time I did it, clearing the files doesn't seem to allow me to start "fresh".
My original thinking was that the Conforming process had not completed, but it sounds like something else might be happening.
First, a few questions please:
What OS, and version, are you running?
Exactly how did you get your MP4 files into PrE - the exact steps might be important?
It's Mac Lion (10.7.5). I pretty much always drag-and-drop files into the media area, but today I've been using the "Get Media" method to try and sort it out. I've just exported the problem files back to mp4 using another application, re-imported, and the problem disappears. This is a similar problem to my other post which I shall now answer...