When I play certain .MOV files, I get video but no sound, only the video.

I am running the latest Quicktime Player (7.7.2) on Windows 7 64 bit. They work with the VLC Media Player but not QuickTime.

The first step here is to determine which audio codecs are not being played. Since the files open and play correctly in the VLC player, use the VLC "Media Information..." window to list the audio codecs not natively playback compatible with your QT 7 Player.

The audio codec (according to VLC) is MPEG AAC (m4a). Where do I download this from for Quicktime Player?

AAC audio is natively supported by the QT player unless 1) the audio is copy protected and your iTunes app does not recognize your system as being a registered user device for purchased file(s) or 2) the AAC track contains an unacceptable data rate/sampling rate combination.

Quicktime Player recognises the codec as "aacl" on the bad (i.e. soundless) file. On the good file (i.e. sound works) it sees "aac".

AACL may refer to AAC low delay or AAC low complexity audio. Problem may be that QT does not recognize the AACL audio tag as being AAC ISO compatible and therefore refuses to play the audio. Would have to run tests on a sample file to learn more. Or, if you want to play around with it more yourself, you could try either using VLC to try and pass through the video and recompress the audio to correct the audio tag or else try changing the file's audio tag to see if QT would then play the current audio data as it is.

How what I go about changing the file's audio tag?

I would probably try the somewhat old "Dumpster" app if I were going to try this approach.