The cheapness of second hand GBA models, backwards compatibility and said backwards compatible devices often being better means we are still somewhat figuring out the common failure modes of such devices. Sound failures being some of the least common and least explored here (charging not happening, screen not working, buttons not work -- dealbreakers one and all, sound less commonly seen and less concerning for many). To that end this is more along the lines of general devices than some kind of "oh it is pretty much always this".
Does the sound go to nothing at all on the relevant end of the wheel? In other audio devices no sound and then topping out low/starting out low and staying there would speak to a iffy resistor (and which point you clean it, your IPA bath likely not penetrating the internals of the wheel/slider/whatever -- on other devices I do usually have to get inside it and go from there or just replace if that is not an option, the sealing that aims to prevent dust from getting in usually meaning nothing else is getting in without breaking it or it being 10 hours to do that and 1 to replace it).
If you still have the board available you can also try sticking a suitable resistor across the relevant pins (I don't have values for either offhand, though should not be too hard to find/figure out) and seeing what goes. Parallel resistance gets fun to calculate but in the end you are still aiming to provide a lower resistance pathway by bridging the relevant pins with a resistor and seeing what goes for the volume. If the volume shoots up accordingly then you have your answer (or the extra force sorted a cold joint between the volume controls and the board).