Bluetooth hands free client volume control
Could you please try below. I did not try myself but sounds similar problem if I understand correctly:
Android Bluetooth Earpiece Volume
"Android is using int 6 for bluetooth volume and not documented. Just use 6 instead of AudioManager.STREAM_VOICE_CALL and try, It should work"
I assume for you will be to replace AudioManager.STREAM_BLUETOOTH_SCO with 6.
Good luck, I hope it will work.