I've got a Samsung Galaxy Note 3 phone and recently noticed that if I'm trying to use the Navigation while Bluetooth is on...Then there's no Nav voice to tell me what to do.
The fix (well, really a workaround) I've found is to install a free app called BTMono (
https://play.google.com/store/apps/details?id=com.bedoig.BTmono)
When you turn it on, the car audio system immediately detects the change and put itself into BT Speaker mode.
It's a workaround because now the car audio system is in BT Speaker only mode, playing ONLY the audio from the Nav...But the good part is the Nav voice is not only back, but playing through the audio system, so it's likely to be clearer/louder vs. from just the phone.
HTH.