I'm fairly certain I found out why I have no sound. I found a service manual for my head unit that has full schematics which helped. The audio ouput from the navi component of the unit is routed to the main board, but there's also a serial connection between the 2. My guess is that the custom igo program that it came with, 'tells' the main unit that there is about to be some sound from navi part, and mutes (or attenuates) the audio that is playing at the time and lets the navi voice come through. The standard iGO/Primo apps that i've tried to run, don't have these extra functions to tell the main unit to let it play through.

If I was motivated enough I'd build a small circuit that tapped into the audio channels, that would mute the main source automatically when a signal was detected from the navi audio output. But as I'm only going to sell this particular head unit anyway (for an upgrade) I don't think it's worth it.

I think the same thing would've been happening with murty, but the intermittent one is probably something different. Hope this helps anyway.