Twin famicom only playing sound in disk system mode

[ATARI7800]

Hello all got my self a twin famicom that was working just fine. However today when I went to use it there was no sound, but when i switched it to disk system mode, the audio plays just fine. Any ideas on what it might be? Thanks 

[zmaster18]

My guess is that perhaps the pin for audio in the cartridge slot could be dirty/not making contact.