From an article: Many of today's modern BMW's contain an adjustment within the radio that allows the user to adjust the speed sensitive volume of the audio system.
Here's what that means:
As the vehicle RPM's increase, the radio automatically does two things:
Increases the volume of the radio
Decreases the low-end frequency response in order to minimize distortion at higher volumes
This is done by BMW to preserve the audio system and to minimize the likelihood that a speaker will become damaged or an amplifier channel will go bad. But, since you're a responsible, BSW listener now, this isn't going to be an issue in your BMW.
So, to learn how you turn the speed sensitive volume essentially "off" in your BMW, see the attached BMW file. You're going to enter the radio service mode, and scroll through the options until you see "speed volume". Adjust this to "1". This is the minimal setting and ensures that the least amount of equalization will occur to your music.
But i know fo sure it has sort of a synthetic add in rev!