Why does my MP4 audio streaming play very low on car my radio?

[Windows Central Question]

MP4 audio streaming plays very low on car radio

I own L1520 which paired okay with Alpine CDE - 173BT car receiver. Bluetooth works perfect including Cortana. Problem is only when I want to play MP4 videos and have the audio use bluetooth pairing to connect to the car receiver; the sound is terribly low even when I've maxed the volume on both the phone and the radio yet MP3 files play great.