Do you have audio ducking turned on in NVDA? That may be the issue. Now, having said that, I'll be darned if I can find that setting, and I could swear it exists, but maybe I'm conflating something from JAWS.
If that exists, and you find it and changing it doesn't fix anything, consider using the Silenzio utility instead of the add-on.
Brian - Windows 10 Pro, 64-Bit, Version 2004, Build 19041
If you think that you can think about a thing, inextricably attached to something else, without thinking of the thing it is attached to, then you have a legal mind.
~ Thomas Reed Powell