There's no volume control for SPDIF outputs as far as I know.. I have SPDIF onto a 5.1/DTS amp at home and no mater what I do the volume of analog lines is always kept the same... that's because the sound card can't mix the volumes, digitalize it and send it out onto the SPDIF ... in fact, from what I know, only digital wave data from the PC can go out onto the SPDIF output, so my question is: how do you have the audio plugged in first place?
Now to the real point: I can't possibly detect when iGuidance is speaking directions.. the only map I can detect the directioins from is Mappoint (because RR sends them). So basically that reduce volume on GPS is ONLY for Mappoint. Map Monkey has its own controls for volume (so RR does nothing for it).. the only way you could possible get volumes to reduce with iGuandance is if you get iGuidance to reduce the appropriate volume (I don't know that it has such option)..