A friend of mine who is a retired sound engineer came up with a possible answer. The older videos had a very low sampling rate of 22 hertz. The newer videos had sampling rates of 33 or 44. My friend thinks that the latest version of the QuickTime plugin may simply not recognise the videos with low audio sampling rates. Does this sound plausible?
Since certain combinations of audio compression, data rate, and sampling rate may create problems, the basic idea does, on the surface, sound very plausible. Unfortunately, when I tested your friend's hypothesis in practice, it didn't "float."
Your audio tracks are IMA 4:1 (PCM) audio having target data rates of 192 Kbps and sampling rates of 22.050 KHz which appear to be fully supported by all media players in which I tested them—to include QT X, QT 7, VLC, MPlayerX, NicePlayer, etc. I then sourced the two sampled files as MP4 files by passing the original video through to MP4 file containers and converted the IMA 4:1 stereo audio converted to AAC audio. In this case I had to lower the target data rate to 64 Kbps since QT does not allow AAC stereo conversion to a 20.050 KHz sampling rate using a target data rate of 192 Kbps. These files where then placed on one of my personal web site pages and re-tested. All 4 files played correctly with audio using browsers forced to play the files using the QT 7 browser plug-in.
Since both your original MOV files and the modified MP4 files play correctly on my web page, I suspect your playback problem is more likely to be a lack of browser playback support for the IMA 4:1 audio compression format due to the manner in which the referenced "VisionaryPictures" web pages are coded for playback. If possible, suggest you replace the MOV files employing the legacy audio compression format with the more modern MP4 H.264/AAC files to see if the audio content is then correctly handled by the page code. All tested files are available for download at http://downloads.walker4.me/Temporary.html
