The Game Boy Pocket is a great little device. The audio is much like that of a DMG, being fairly rich and bassy. With an external power supply, though, there is a nasty hum sound that is very loud. This modification removes the hum sound as well as helping the audio quality a lot. It’s by far one of the simplest game boy mods available.
You will need:
-2 x ~4" of wire (strands of PATA or Floppy ribbon cables are perfect)
-A bit of solder
-Soldering iron
-Game Boy Pocket
-Tri-wing screwdriver
I am assuming:
-You have a Game Boy Pocket
-You know a bit about super basic soldering
So, first get out your game boy pocket.

Flip it over, and remove the 6 screws.
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Take off the back shell.

Now, take a look-see at the bottom.

See the little things labeled EM2 and EM3? Remove them. You can just take your soldering iron with some solder and just push down on them and slide them away.

Now turn your attention to the volume dial towards the top. This is where we will take our new audio source from.

Solder your wires to Pin #2 and Pin #3 (from the top down). Pin 2 is Left audio, Pin 3 is Right audio.

GO back down to the bottom with your wires, and solder them to pins 2 and 3 on the headphone jack. 2 is left, 3 is right.

Here is how it will end up:

Now, put back the shell, screw it together, and enjoy!