[joeschmoe]

I hate to say it...but have you checked the obvious?? Volume turned up on the GPS?
My Zumo 550 has to be turned to MAX volume for me to hear it come through the speakers of the radio, might verify that your volume is at MAX....
The interconnection from the GPS to the sound system is VERY basic...Connedctor under the console plugs GPS into Vision wiring harness, then wiring harness runs back to audio unit....That's it...