I have been looking around about this same issue and found something I thought was interesting. Peripheral makes a pre-amp boost that I believe could be connected between the nav wires and the inputs on an amp. It has a gain control from what I see that could be used to adjust the volume of the voice to a desired level. What do you think?I setup the unit a simple way. It will not mute the audio, but the lady voice will be there, when you activated the Nav.
Here is how:
When you removed the head unit, you would find a harness which had 4 wires on it. These wires were coming from your Nav.
Take the Lavender/Black (+) and white/Black (-) from that harness and connect them to your input amps or powered speaker.
I suggest you to get the SRK-5 Clarion centre speaker with built-in amplifier for louder voice and easy installation. You can purchase this speaker for around $85.00.
Good Luck !