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For what it's worth, I went with an op-amp style (Cmoy-inspired) solution. First attempt was using a home-made board but I have now made an SMD version that is a good deal smaller. I like using an op-amp because it would sacrifice itself before you end up doing any damage to your CPU itself (say due to static from plugging in an audio jack or something).

The downside is that it needs power. My first design required a separate power brick but I found a low voltage op-amp from Texas Instruments (Burr-Brown) that can be fed with low enough voltage that I can just use one power brick into the NES and tap off those inputs. That requires using a DC power brick though (the original NES power brick is AC), but a nice clean linear regulated power brick should work just fine, if not better, than the original power brick anyway.

You can see my first attempt here:

http://www.moocowproductions.org/nesa/

Still waiting on parts for my new board so I can't claim success for it just yet.

I finally broke down and tried disconnecting the backlight LEDs, but it didn't appear to make a difference. I suppose the next thing to try is swapping out components other than capacitors (I've already replaced all of the electrolytics), such as Q1?

Wow well the problem instantly went away as soon as I blanked the screen in mGB. Sounds like awesome now. But since the LEDs are still active, that means there's less of a chance it's the LEDs, or is it?

As in, now that I know screen blanking works...it doesn't seem like there is much I can do about that?

EDIT: At least until I start using mGB Which still requires I finish up my ArduinoBoy (it's taking a break right now whilst I work on my MidiBox stuff)

So I tried changing the contrast and the ROM. The contrast only helped very marginally. The ROM also didn't change the high pitched noise in any of the screen configurations that it cycles through. The patterned screen does create more noise than the others, but it sounds more like an addition to high pitched noise itself. I will try removing the Pro Sound mod today but will have to wait until this afternoon most likely.

I tried the ROM on my other GB as well and you can faintly hear a high pitched whine on what I think is the patterned screen. I can't tell because I have to put my ear right next to the speaker and can't see the screen Otherwise the normal noise from my good GB is basically constant (mostly some hiss and a faint buzz).

I am still banking on interference being the potential cause, though.

For what it's worth, I went ahead and gave one of my DMG-001's (the noisest one) a cap kit and, to my surprise, it didn't seem to improve things music. That said, I listened to your recording and I don't hear anything abnormal except that noisy bit right before the square. I think that might not be much to worry about.

By contrast, my GameBoy emits a high pitched whine that is independent of the volume knob and it drives me nuts! My other DMG has that same high pitched noise but it's much much quieter. It's a bummer since that's also my GB with the backlight mod.

In any case, I would try yours out with LSDJ, which has an option to control power save mode and can give you a better idea of how noisy things are. In lieu of that, if you have a game that has spare music or music at a lower volume, you can better judge things.

I would grab a cap-kit from Kitsch and give that a try. I have one ready to go for one of my GameBoy's but haven't sat down and done it just yet since I have another GameBoy I use for recording anyway. Between those two, the difference is massive. The bad one has a really annoying high pitched whine that is difficult to fix in post, whereas the other sounds like silk. Well, ok, it too has a very very slight whine but it seems to be at a different pitch and seems to be basically inaudible when I record it after EQing.