stereo goes to "single play"

[doo71dene]

i have a 2002 eb, every time i start it up the stereo is in "single play" mode, as to say it sounds like an echo, i have to reach back to the rear controls and put it back to "dual play" for it to sound normal. it hasn't always done this, so does anybody know what button might have been pushed to cause this? i have kids that like to play with controls in the back by the way.