What about taking more of a hardware approach to the problem of muting the music when nav instructions are given? I use a laptop for my CarPC, which has a built-in sound card, and I also have an Audigy ZS Notebook card in the PCMCIA slot - so I have two sound cards.
I've read that you can configure WinAmp (which is used by RoadRunner) to select which sound card it uses. I'm wondering if I can set WinAmp to use one sound card, and then have iGuidance use the other. This way I'll have two RCA cables, each with different app audio output. Then I can use an audio switcher device like
this to automatically switch the output being sent to the car amplifier when any sound is received from the iGuidance output cable, and when it's done, switch back to the WinAmp cable.
I'm not sure this will work yet, but I thought I'd float it here and see what anyone thinks.