My car has Bluetooth for making / receiving calls. It also allows media playback if I switch to Bluetooth instead of USB. I use my phone for sat nav so I'd like to be able to hear the audio telling me which direction to go and of any traffic problems. The issue is that it routes that audio to the car and I can only hear it if I switch the stereo to Bluetooth media, as if I was going to play songs from the phone. That means I either can't listen to music on my USB or I can't hear the sat nav. Is there a way to get my phone to play the audio back through its speakers in n steal of through the Bluetooth but still allow phone calls through Bluetooth?
This seems to be the setting -- go to Maps' Settings (tap the three lines in the left top corner to open a pane, then tap Settings there), there go to "Navigation Settings" and turn off "Play voice over Bluetooth" switch.
You could try Sound About, available on the Play Store. It has settings more specific than your phone's stock utility for routing audio through many devices, stereo and mono. Using it on my LG Stylo 2. Works well.
On my Samsung Galaxy S4 (Lollipop) using Samsung Music (recently updated, but this was the case when I bought the car, before the Lollipop update, and immediately after that update as well), with a 2015 Ford Fiesta (basic Sync Bluetooth audio and controls), this is the default. When I'm playing music, either Verizon Navigator or Google Maps directions (if set to give voice directions) pauses music playback to speak through the car's sound system. Even if I'm listening to FM radio, the broadcast is interrupted for directions. If I'm on a phone call, the directions voice is mixed, much softer than the voice of the person I'm speaking to (but still there).
Other phone initiated sounds (notification, for instance) also pause music, though keyboard sounds don't. Car initiated sounds (low fuel warning, service required warning) pause playback as well.
If this isn't happening with your phone and your car's Bluetooth, I'd start with checking your phone's general settings, then those of your navigation app, and finally those of your car's Bluetooth and sound system. It's got to be in there somewhere.