This may or may not help since I am waiting for arrival of my new Sunseeker. But, in my car, I had this same problem.
Turns out, at least in my car, there are two "volume" controls but with one knob. When the radio/CD is playing, the volume control knob controls the volume as you would expect. When the voice navigation would come on, the volume knob controls the sound level of the nav system. I felt silly when I took my car into the dealer to fix the nav system and the service writer showed me how it worked. My volume for the nav system was turned all the way down.
Don't know if that's how the Jensen system works, but it won't hurt to try.
Out West Somewhere
2016 Sunseeker 3050DS towing 2006 Hummer H3