Oh, cool, thanks for the new information. I didn't mean to get all about the semantics on you a minute earlier, sorry.

The whole setup is care of LOOMIS, and I honestly haven't looked inside yet, as I just got it and have been working on my own DMG mods. To my understanding its the v3 backlight from kitsch (could be wrong) and an internal line-out sound mod, as it uses the already present jack. To reiterate and to hopefully alleviate any more confusion, the backlight doesn't waver or dim, the sound works just fine, the battery indicator LED doesn't dim or waver, only the intensity of the LCD itself. It gets weak when music plays. Having just finished my own backlight and sound modding last night, I can say that this wavering/dimming only occurs on the DMG I haven't had my hands in.

Any other mods on the unit? Also, who modded it?

Mincing words, yes, the LCD's contrast wavers. I suppose it doesn't dim as this could suggest a source of light is becoming weaker, where in fact there is none. Only a wavering LCD contrast.

I don't really think the LCD "dims" in the sense of the word I'm understanding you to be using right now.

Maybe something makes the contrast waver a bit, but there's no light component in the screen other than the added backlight that could be "dimming."

Would this still be the issue with the LCD dimming, not the backlight?

I agree with Mr. Maize. Try the left most leg on Q1 on the back. Also, the same effect has happened to me when batteries get really low.

Sounds like backlight is not on a regulated source

Mind you the following is just me assuming based on what I've read, not from experience.
I haven't modded one myself yet, but I know there are two ways to draw power, directly from the batteries or via the power regulator. There are some mods with lights that recommend soldering to the regulated power source, so it could be that what you have has not been soldered to that.

So.. backlit screen, when buttons pressed or sound plays the LCD seems to dim. Really annoying when you're writing tunes. What might be causing this? This gameboy came into my hands sound modded and backlit, so I haven't had my hands inside of it. If anyone is familiar with this, let me know what helped you, or point me in a certain direction.

EDIT: Replacing with new batteries helped, but not much. Seems to still be drawing power in a weird way.