No Sound from QuickTime 7

I'm using QuickTime 7.6.6 on El Capitan 10.11.5. Either there is no sound or sound for a moment or two playing my old mov. movies. There is no problem playing newer movies or MP4 movies. There old movies played fine for years and years. Why not now? How can I fix the problem?

What codecs were used to create the MOV files? If legacy or proprietary codecs were used, then that might explain why they are not now supported under El Capitan. Fixing the problem would depend on the codecs used and the software you plan/want to use to convert the non-playing files.

I've partly solved my own problem. Open the old .mov file in QuickTime 10.4 and save it as "Converted." Open the "Converted" file in QuickTime 7 and the sound is back, but not every time. What is really interesting is that QuickTime 10.4 will not open the "Converted" file. Strange.

Again, the problem depends on the audio codecs used and whether or not they are supported. If your system's codec configuration supports the audio codec, then the converted files would probably be playable. Other codecs (like DTS) simply can't be converted using the QT X Modernizer/Convert structures embedded in El Capitan. Either upload a problem file for download and analysis or use a utility like MediaInfo to determine specific problems. If all you want to do is convert the files, then I would recommend using the free HandBrake app. It employs built-in codecs that support most common compression formats and doesn't rely on the QT X El Capitan embedded structures or QT 7 codec configuration for conversion.