Really I'd have settled for the other way around... grin.
I'm not sure this is possible automatically unless there is a specific program which runs when it speaks, in which case one can use profiles. However if you want navigation on that dialogue to be spoken, I do not see how you can silence nvda and still keep the screenreader functions other than manually by hitting control.
Brian

bglists@...
Sent via blueyonder.
Please address personal email to:-
briang1@..., putting 'Brian Gaff'
in the display name field.