For headphones then European models (and later world models) would have a nice little resistor in to limit the current from the headphone port to save the hearing of the kiddies when they are using in ear headphones. I doubt there will be anything like that here for an internal speaker but I have been surprised in the past.
To this end you are left with either replacing the speaker, and possibly the DAC or adding an amplifier circuit somewhere in the mix, or tapping the headphone feed and feeding that to an amp and onto another speaker you attack to the back or sides or something. The latter not having the issues of space and probably able to be transplanted easily between GBAs where messing with the DAC and adding an internal amplifier will involve messing with the GBA PCB and finding space inside it. I don't think the GBA ports have a power thing you can tap to power your new external speaker and amp circuit so you might then be left using another battery* or trying to tap the GBA SP's own battery to do it (could do it with a ribbon connector that sneaks down past the plate if you wanted)
*easy enough to get a nice USB charging one these days which could also function as a battery booster for the SP if you wanted.