This appears to be the same problem I experience with NXT6. I have spent MONTHS with tech support. All of their suggestions are WORTHLESS. I even created screen captures to prove what is going on. They (tech support) continually feign ignorance to attempt to close the inquiry number. Interestingly, I finally came across one tech who mentioned this is a common complaint.
Here is what I discovered - for whatever reason when the program creates the audio proxy file, it creates a “corrupted” proxy audio file. The longer the video to be edited, the more fuzzy/corrupt the sound will progressively become. It is clearly a problem with how the program extracts audio from the video when it creates the proxy file. They may have hard coded an improper setting into the program or something. You can confirm this yourself by changing the default audio proxy directory in settings and then listening to the audio file it creates. I can’t figure out if the program uses the proxy file for rendering or if it creates another temporary (yet also corrupt) audio file.
This is repeatable on multiple machines. An i9 - MacBook running windows duel boot - even my surface 4. All of them exhibit this behavior. I believe they use ffmpeg to extract, but I still don't know how it is called up for the conversation.
This is entirely due to a poorly crafted program. I can replicate the problem with multiple machines and differing files.
It is not your drivers (video or sound) - it is NOT a machine dependent problem. It is a bug that Roxio/Corel must address.
I was hoping a fix would be issued by Roxio/Corel to take care of it.
Contact tech support via email in this site. Let them log a complaint. https://www.roxio.com/en/support/