If they really are MPEG-4 as you say, the problem might be that the audio is something QT doesn't like in mp4 files, like mp3. You can convert those with ffmpeg, or if you like GUIs, VLC (use Open File... from the file menu, then check Advanced output). But I suspect your files are MPEG-1 instead of MPEG-4, in which case MPEG Streamclip is the best solution.