I think I have found a bug in Android 6.0.1 on my Samsung Galaxy S7 Edge.

Put all four volume bars (Ringtone, Media, Notifications, System) at full.

Connect your phone to your car via. Bluetooth.

Put some music on (any app will do, they should all work the same).

Make sure all four volume bars are still full.

Open Google Maps and make the voice say something.

On my phone, the Media volume (that controls both music and voice notifications) turns down to around 50/60% every time s/he speaks.

It's very annoying!