Are you receiving sound through your headphones but no sound will come from the main speaker?
Most likely this is due to the oxidization of the headphone jack. This should apply for all Game Boy Consoles.
Pin 5 should make contact with Pin 4 when there is no headphone jack inserted. Oxidization over time can cover the pins causing those two pins to not connect, which causes the speaker to not function.
How do I fix this?
Simply insert a headphone plug into the jack, to separate the lever and pin, then clean with IPA and this should fix the issue.
For a more visual idea of this description, please scrub to 27 minutes in this video:
Information about how the audio works on the motherboard begins at 23 minutes.
Thanks to Luke at RetroSix for the visual information!