If you're on iOS and you have "adjust volume with buttons" enabled, try this: 1) turn on the voice commands feature; 2) go back to the map; 3) hover your hand over the proximity sensor to activate voice commands; 4) While it is waiting for your command, try adjusting the volume all the way up.
If it is all the way up already, I think we've reached the limit of my help. You may try adjusting the volume of the music down, but I think that will affect Waze volume, too. I don't have any trouble with the mix via BT on my car, so I would think it would be the same...