Seeing as this is a useful contribution of information, adding this to the scan thread is fine!
What i found was on the pins 1-5 for the headphone jack.
1st, The common or ground on headphones do not connect to the ground on the GBA.
2nd, Only while the headphone jack is on the PCB and nothing is plugged in, Pin 4 and Pin 5 connect.
3rd, When you plug something into the headphone jack, it disconnects Pin 4 from Pin 5
Pin out
Pin 1, Headphone common, not GBA common.
Pin 2, Headphone Left stereo speaker, or White RCA out.
Pin 3, Headphone Right stereo speaker, or Red RCA out.
Pin 4, GBA Common/Ground, does not output through the headphone jack.
Pin 5,A, While nothing is plugged into the headphone jack is connected to Pin 4
B, Turns off the onboard speaker if something plugs into the headphone jack.
C, If you remove the headphone jack, the onboard speaker will not work without a jumper from Pin 4 to Pin 5