Hi BonzoMAC,
Thanks for using Apple Support Communities! I see you're having an issue playing some files with Quicktime, and I'd like to do what I can to help.
Have you restarted your Mac since noticing this issue? If not, go ahead and do that, as a restart can resolve unusual issues.
Check that you're on the latest macOS Catalina version which is 10.15.2. If you need to update, back up your Mac first, if you haven't recently.
Back up your Mac
How to update the software on your Mac
If you're still having the issue, then see if it happens with another user. How to test an issue in another user account on your Mac
If an audio or video file doesn't play on Mac may also be helpful.
If you're still having issues, I'd suggest contacting Apple Support.
Here's some additional info on QuickTime: Open and play a file in QuickTime Player on Mac
Take care!