For me, I have had ducking turn down sounds when I didn't want it to do so, including nvda, I have had things duck for no good reason and staying like that.
What does this even mean? I'll need further explanation to find out if this truly makes sense or not. Here's how ducking works, at least NVDA's two ways of doing it. Audio ducking involves lowering the volume of other audio. NVDA may be set to lower the volume of other audio when it's speaking, or generally when it's running. The shortcut to toggle this behaviour is NVDA+Shift+D. Now, ducking does not turn down NVDA's speech or the sounds it makes, never has and never will.
Unless, of course, you're talking about the effects that come with your version of Windows or laptop, which can messed with NVDA's sounds. In that case, that is not the fault of NVDA, nor is it a feature. Note also, that audio ducking is off by default, so someone who really wants it, like me, turns it on by choice.
Finally, some synthesizers for NVDA may not have good support for audio ducking, as they lower the volume of the sounds and sometimes, if set to only when NVDA is speaking, keep the volume lowered. I'm talking about the illegal add-ons that are all over the place. That, again, is not the fault of NVDA nor the feature, but the add-on itself.
Please clarify what you are saying!