Something to consider is that switching to aux for voice prompts is probably going to be pretty jarring. I suspect it'd work better if you could get the "mute line" created and just use it to trigger the hush function. This is how OEM nav systems work, and it's a much more pleasant experience.

I use the mute line to switch to aux for the phone, and it works for that great, but I can't imagine having it pause the music and say something every thirty secconds as I approach a turn.

Matthew