This thread contains my work around:
If I go into tuner, I peg the volume for Sirius. Then I go to source and select Bluetooth. There seem to be some songs, coming from somewhere, ready to play, but they are not playing, and I don't want them to play. Now, If I work my way through the menus to the erstwhile grayed out volume choices, nav voice volume and Bluetooth are active. I go into each of those and max them out (11/11 and 6/6 respectively). At this point, a loud, adjustable Nav voice is produced.
Lastly, I suspect that this is something that benefits from storing the key fob settings.
I have to go into Sirius and split my ear drums maxing out the volume, got to source and switch to blue tooth, and I get the volume back. This is a PITA.
I can also go to source, select Blue Tooth, and max out the volume in Blue Tooth. And even though some song is identified and seems to be trying to play, it is not - no music comes out, but the volume is maxed. Then, I've got volume in the nav voice.