Heres an idea, have GMPC running in the foreground with nRoute running in the background.
That way you get to watch GMPC on the screen and listen to nroute on your speakers.
This also has the added bonus of; if GMPC crashes, nRoute (or maybe iGuidance) is already running in the back ground.
I'm not saying that GMPC is likely to crash, it's just that the only way I know of using multiple programs W/ one receiver and GMPC, requires the use of the software xport v1.21 (on the rear end of GPSgate), which likes to just stop working or crash my system, it's like it wanders off to look at a deer, sometimes it comes back in a few seconds or just dies all together, on a few occasions it blue screens.
On another note I have GMPC running on a 2002 HP laptop w/ no volume problems what so ever.