MacBook Pro 15" Retina Catalina mid 2014 QuickTime issues
If the problem only occurs in one application, then it's likely to be a problem with its settings, or some system setting that controls audio per-application.

Wiping the drive and then putting all the files back using Time Machine will leave you with an identical drive.

You may want to try deleting the file at:

~/Library/Containers/com.apple.QuickTimePlayerX/Data/Library/Preferences/com.apple.QuickTimePlayerX.plist


(i.e. in the User Library.)

... and then test the problem. If that doesn't work, you may want to restore this file to keep existing preferences.