I have also had this same problem but not specifically music from the USB. It seems like the volume for the calls/voice commands, isn't separate from the volume for the media (which it really really needs to be). Also, when having the car read a text message to you, it doesn't mute/pause the music, and the volumes are not independent . "Hey, I can't hear my text message over the music, let me turn up the voice volume.. oh now my music is just really loud".