Hardware Noise at high brightness using earphones and low volume on my GBA, how to fix?

[KOTYA]

I have a AGB-101 that I have modded last week with FunnyPlaying IPSv2 screen.
It has the same low volume issue on speaker even after I modded it. So the cause is not the screen mod.
How do I fix this? Do I replace the potentiometer? Reflow the AMP AGB chip? Install a third party amp? Replace the speaker? Is there a way for me to short some pins to identify of the potentiometer is the problem?

Since my speaker volume at max volume is barely audible at arm length, I've been using earphones.
I noticed another issue when using earphones since the IPS mod. There's a screeching/piercing distortion when using the screen at high brightness (level 5 to 10. No such noise at level 1-4 however).
Since I'm bad at explaining things. I have included a 30 seconds video to demonstrate both issues. How do I fix this?

 

[FAST6191]

LEDs don't tend to change brightness by themselves. To that end people tend to use something called pulse width modulation (PWM) aka turn the thing on and off fast enough and keep it in a state of dimming as things don't immediately leap to full brightness. If your camera there has a high framerate mode then have a look at that for the various brightness levels and you might be able to see it.
I can't be bothered to download and run an analysis on the audio but judging by the sound and differences in pitch when you are cycling through brightnesses then yeah.

You either need to isolate the screen better (there is a reason your TV or something might have a little ferrite ring on the power cable, and laptop power cables have a block a few centimetres from the connector end, or VGA cables had similar things). I don't know where the noise is coming into the system here (would have to test) but the usual suspects are the point you took the extra power for the screen light, or the screen itself inducing noise in the item below it. For the former then some kind of coil or ferrite bead, assuming you don't have a better solder point further away from the audio circuit. For the latter then it can be as simple as putting a sheet of aluminium foil or something between the screen and the rest of the board and maybe tying that to ground on the circuit.
Maybe you could isolate the sound/do some kind of pass filter on it, though this is the weaker solution and might mess with the GBA audio quality at some level (if you knock out the high pitched sounds you also knock out any good/wanted high pitched sounds).

Loudness if it happened before mods sounds more like a dodgy potentiometer/wheel thing (not seen as much of this on the GBA but it is a thing in any number of other devices). There was some discussion of replacements in a thread the other day (seems some of the GB/GBC stuff can be adapted). You should be able to short it out (though I would use a resistor rather than a nice thick chunk of wire -- it will likely have a series resistor in there as a volume limiter anyway but shorting such things is a dubious prospect, though as a test rather than permanent mod I guess it is not the )

If you want to do a third party amp then that is an option, though unless it has its own internal filtering I would not bet on it curing your noise issue. Not sure what goes for improved quality/loudness speaker replacements these days (most usually found suitable ones in another device and went with that).