Headphones Working But Speaker Is Not
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 results in a non-functioning speaker.
Simply insert a headphone plug into the jack, to separate the lever and pin. Then clean with 97% or higher 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 RetroSix for the visual information!
If the issue persists, consider replacing your speaker with a new one, especially if it is the original speaker.
Feel free to reach out to email@example.com for further assistence.