With the 1.47 install default of "8", the music would duck and the NAV prompts would play, but just not loud enough to be heard. Once I started increasing the NAVSOUNDLEVEL, the ducking stopped happening. So what I'm getting now is that "110" is actually "10" units LOUDER than the current music, which lets the prompts be heard.

But, my guess is what you wanted to happen was the music to duck, then the NAV prompt to be a given level, then the music to fade back up. That WAS the cycle when NAVSOUNDLEVEL was set to 8, but it's not any more.