I have had the same problem! Jerry 745Li has explained the reason perfectly!
I was going to say try cleaning the disc if its been in there ages, but you say its a new one, Have you got an original disc to try?
In the mean time remove the NAV and loop out the fibre connectors, i used a stiff bit of wire, a short rubber tube 3mm internal diameter and some tape! sorts out the audio issue!
Try pulling the power from the NAV to reset it.