Sound in mp4 videos are missing?

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Try installing Perian from http://perian.org/

You should read this support page http://perian.org/#support in case you need to delete older codecs. In Macintosh HD/Library/Quicktime/ delete any files relating to DivX (Perian already has them). However it should be noted that Perian is not an internet plugin and will not play DivX files imbedded on a website. For that you will need the DivX Player browser plugin available from http://www.divx.com/divx/mac/

Are you using Quicktime 7 to play the videos?

Snow Leopard has two versions of Quicktime:

Mac OS X 10.6 includes QuickTime versions 10.0 and 7.6.3. The QuickTime 7 player will only be present if a QuickTime Pro key was present at the time of installation, or if specified as part of a custom install, or individually downloaded.

http://support.apple.com/kb/dl923

Snow Leopard update 10.6.4 included an update to 7.6.6 (if installed). You can install it from the above link even though it says for 10.6.3. It's the same version of QuickTime Player 7.6.6.

Only QuickTime Player 7.6.3 or 7.6.6 can be updated to "Pro".

Try using Quicktime 7 to open those files.

I have the same exact problem with a Mac Mini running 10.6.4. I converted some of my DVDs to m4v and mp4 files. The video is fine but I have no sound when I play them in Quicktime, iTunes, and VLC. However when I put these same files on my older Mac Mini running 10.5.8 the files are fine and have sound in all the applications. This tells me there is nothing wrong with the files. It is strange in that I recently created another m4v file from a digital camera movie and it plays fine on the newer Mini running 10.6.4. I used Handbrake to create the m4vs from the DVDs. Could this be another Snow Leopard glitch? I think these files were fine until I upgraded to 10.6.4.