.MOV files are pretty generic container formats, they can have almost any form of audio and/or video formats within them. With this in mind, it's probable that your files either use an uncommon format, or one which is not officially supported in .MOV files.
Many players will ignore whether or not a given format is supposed to be within a given container and go ahead and play it so long as they have the codecs for that format. Some players on the other hand will only play back a format if it is a "legitimate" format to be within that container. I did a quick websearch on your issue & found a few posts elsewhere about people having the inverse problem (video but no audio), and their problem turned out to be an unsupported audio format, so there's a good chance your issue is simila.
As a step to solving the problem, you should open the video with an information program to see what format it's using. Mediainfo is one of the best such:
http://mediainfo.sourceforge.net/en/Download/Windows