As I understand it the nav audio goes through the TV module and on to the radio unit, using shared audio input pins there. I think the radio switches to the nav audio mix upon receiving a command on navbus. It is my opinion that the audio is connected correctly between the nav, TV and radio module, since they work on TV mode.

When you are expecting nav audio on CD/FM, do you get the expected audio mutes/fades but no voice prompts?

Are you using a BM24 or BM54 radio? If the latter is pin 9, 5 and 6 on socket C fully connected all the way to the GPS module?