Here are some illustrations below of adapting a PAM8302A Class D mono amp board in to your build. Its a perfect and cost effective way to add amplification to your gameboy macro build especially if you are only concerned with playing GBA games.
These boards from adafruit are widely available too
The additional speaker has been daisy chained in this instance, you can do without but this is down to user preference
I have tried to provide as much illustrative detail so it can be presented in its simplest form.
Always take care when performing any modifications.
All illustrations above are created and provided for your reference by Obirux.