Alpine cde-133bt

Ok, here goes. I had an alpine cde-133bt head unit in my explorer for about 6 months. Couldn't stand the looks of it so i took it out and put my old head unit back in. It worked perfectly fine while i had it installed. I have now given it to a friend in exchange for helping me do some work on my truck. He installed it in his 98 VW bug. It was working fine on his way home and all of a sudden the sound quit. Everything seems to be working fine except no sound. He has no amps or any added on equipment. It's just wired into the car with a kit from American international to the factory wiring. Anybody have any ideas as to what could be the problem? Any help would be much appreciated.

thanks

ryan