Just one thing you have turned the Navigation up haven't you....

Course you have.
Try this, Bluetooth you smart phone to the Nav 5 then pair your ear buds to the phone--- if that does not work try keeping your phone attached and pair your ear buds to the Nav 5.
I've spent many unhappy hours trying to get my Sena headsets to talk to the Nav 5 both for directions and music and talk to my pillion, which is still a work in progress, the hole Bluetooth thing is a faff.
What with batteries dying and then the Bluetooth has lost connection for no reason at all.
Give me a good old connection any day.
PS: for the past 20 years I've worked in IT and have the patience of a saint when it comes to teck!