Strangest thing happened. I've received a Gameboy Color completely broken, with no audio or video, except a power button. After cleaning, I got some lines and I figured the LCD Potentiometer was broken, so I replaced that, but nothing - and then I replaced the speaker last night and nothing. I completely cleaned the board with IPA and nothing still.

Then, I figured I would test the headphone jack sound. I plugged it in, and turned it on. Full audio and video. I took out the headphones and tried again, and now the speaker works completely fine. From the schematic, it looks like U3 controls sound, but it's connected to the main CPU (U1).

Why would plugging in the jack fix the entire GBC? Could someone with electronics knowledge please explain? Thank you, and happy gaming