I disconnected the component cables and red/white audio cables from the Slingbox 500 and connected the Motorola PVR to the TV using component and red/white cables.
No sound on the TV. So I think the issue is with the analog audio out on the Motorola PVR.
I'm going to Radio Shack to pick up a digital audio to analog converter. I'm then going to try using the digital out on the PVR and connecting it to the analog in on the Slingbox 500 via the converter.
I'd rather try this than getting Verizon out here to replace or fix the PVR box.