This works for me too. My car (a Honda Civic) does support music audio, as well as phone call audio. Works really nicely and as you would expect. If you are playing music through your phone via Bluetooth,, it lowers the sound when giving a verbal navigation cue, and then increases the volume when done.
Audio really works well in the Civic and I have really been pleased with the combination. I actually like the new Apple Maps (I must be one of the few, I guess 🙄) and am really happy to have turn-by-turn navigation natively on my phone.
By the way, the one situation where there does not work is when you are playing a CD. If you get a call when playing a CD, the audio stops and you hear your ring tone and can answer the call. But you can not send the navigation cues and have them output at the same time the CD is playing (not that I have found). You can have them come out of the iPhone speakers though (I haven't tried this yet).