I was wondering if anyone had some ideas on how to amp the internal sound on the gba? I was thinking if I could make a small smd amp.
You could do but from what I remember the space inside the GBA is already pretty tight, so you'd need an amp chip wired to audio in and out (to both speaker and line out) and power, and the amp-amount, plus you'd also need a heatsink unless it generated very little heat.
I would look to modify the existing amp, unless it's a class D driven directly by the 1-bit DAC. That would be clear because the SoC would directly drive the speaker. Anyways, with a separate amp chip, perhaps the volume pot is biased with another resistor that you could get rid of. Or maybe the gain is settable. Or maybe the amp rails come from a charge pump to generate >3V, and you could modify that for a higher voltage.
Pictures are useless as it's got the white coating on it so you can't see where the tracks go. There's VOL, ROUT and LOUT test points on the reverse of the PCB near the audio socket though. Traced the speaker connections back a few times past the SMD resistors but there's too many holes to try and trace. The AGB schematics would be a much easier way to check it.
