I believe I've found out the issue. I think it's the left channel on the pcb itself that doesn't work, because when I connect the left and right together to make it mono, both the built in headphone AND the prosound work, but only in right mono-- if I set anything to play just in the left ear it won't play. I think the only solution is to start new with a new DMG, thanks for all your help, everyone, I guess it wasn't the prosound after all ^^;

Yeah, I've actually done that twice now-- and I've replaced the wires entirely and it's still having the same issues ^^;;

I've tested it with multiple headphones and basically no matter what I do I only get sound in one ear

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I've completely resoldered the prosound module again, and it still is having the same issues. By now I really think it's the prosound unit, unfortunately I don't have the original packaging anymore so I'm just going to buy a new one and hope it comes in time

I removed the prosound mod and the built in audio port still didn't work correctly, I think it might be unrelated?

I should add some important information, I goofed and said in the OP that I'm using a 1/4" prosound kit-- sorry about that. I'm actually using the 1/8" prosound kit, I have no idea why I wrote 1/4", apologies!

I can try this-- but I've been testing the pro sound jack while the case is open and while it's closed-- and in both cases I still encounter the issue of only getting sound in one ear.

From the picture, do you think it's the ground point that's been bridged? It looked fine to me, but I'm very new to soldering ^^;
I've been testing both jacks with audiotechnica ATH-M50 stereo headphones and cvs brand cheap earbuds, both are 3.5mm

That's what I get for skimming. Missed the part where it was causing issues in the built in jack too. I'd throw in a vote for bridged points too.

The modification is affecting the built-in jack as well, which means it's almost certainly an issue of bridged points.

What cable are you plugging into the jack? The issue sounds a lot like plugging a mono 1/4" into a stereo jack.

The quality of your soldering looks okay, but there's too much wire exposed at the contact points; it looks like you kept the iron on there a little too long and the coating on the wire melted off. It probably works fine when the gameboy is disassembled but when you close it it may be pressing the wire against some other contact and introducing a constant voltage into the signal which you're hearing as hum.

Basically you need to undo the solder joints, cut the exposed part of the wire down shorter, and redo the solder joints faster - it might help to apply the iron to the contact on the board, then as soon as the solder flows, apply the wire, then remove the iron. This should all take no more than a second or two for each joint, all told. You might find some junk electronics to practice on first before attempting this again!

Good luck!

Here's a quick picture of the pcb I took (the 5 audio solder points), it doesn't look like anything's bridged as far as I can tell-- I don't have access to an ohmmeter yet but I'll try that as soon as I can

^^ great help!

Ensure that you've soldered to the correct points and that there is no bridging between any of the points. If an ohmmeter shows no resistance between one of the channels and ground, then you've accidentally bridged them together. I've seen this before and I'll bet this is the issue.

My reasoning is that the built in audio jack is having audio channel issues independent of the wiring of the prosound. It's possible that I'm incorrect in this, but I switched the left and right wires for the prosound, and that switched the side that worked for the prosound jack (IE it was right only before, and once I switched it, it became left only), but the built-in audio jack is right-only regardless of the prosound's wiring, which suggests to me it is an independent issue. I'm sort of new to this, but since the built in audio jack is having a consistent issue, and the prosound jack's audio issues change depending on the wire, I think they're separate, anyway.