entertainment system loses volume while playing

[jloftus577]

the rear entertainment system will lose volume after playing for 30/45 minutes.

[marctronixx]

if you want some help over the internet, it helps to be a bit more thorough. ;-)

let me kick off the party..

-is this a new car to you? or is it used?
-did this always happen or did it just start?
-so inside the headphones the volume will lower by itself no matter what the source of the music/video?
-the main car speakers do not do this? just over headphones?
-is the rear system factory installed or aftermarket?
-you can reproduce this no matter the source/input?