pioneer volume problem

hey guys have a 2006 pioneer cd radio that has face graphics in a 1992 honda accord that has a year and a half that I did the install and the customer says the radio volume lowers itself when he presses the foot brake, and when he accelerates the volume goes back to normal after a couple seconds.this is the second customer that tells me the same is it because they changed some setting on the radio or does it need to be reset? or is it a defect? the install is ok no loose connections.what is causing this?thanks in advance.  

The customer probably has the ASL feature turned on. I believe it stands for Audio Speed Level. When the road noise increases (acceleration) the volume goes up, when it lowers (brake) it goes down. There is a low medium and high setting for this or you can just turn it off.