I know my speaker works because I tested it in another GBC, just to be sure that I didn't need to change the speaker. By this point I was fed up with having no sound and I was ready to just bypass the headphones and hardwire the speaker to get any sound from it at all. When I held the speaker wires to the headphone connections on the other side of the board the speaker came to life, which I found a little odd, as I don't believe the headphone jack should be live at this point.
So after a little more poking and fidgeting around, I goofed into figuring out what was ailing my particular GBC. Within the headphone jack of the GBC there is a tiny switch that either opens or closes the electronic signal path to the speaker output. As anytime with metal, sometimes it can corrode or tarnish, as was the case with my GBC. This doesn't allow the switch (simply two pieces of metal) to close properly, sending the signal that should be going to the speaker, instead directly to the headphone jack.
|The switch in question is at the bottom right of the jack.
|Outlined in red and yellow, these two pieces of metal should make
proper contact to send the sound through the speaker.
This connection is absolutely minuscule, making it almost impossible to properly clean between the two bits of metal. I tried to slide a tiny flat screwdriver down between the two and scrape away anything I could, without success. Finally what I did was press the one piece (highlighted in red) down with one screwdriver, while pressing up toward the other piece from a small hole on the side of the headphone jack, located just below the connection point.
After a little more tinkering around I finally got sound. Just to be sure it would work I plugged in my headphones, but once I unplugged them the sound was gone again. After a few more attempts of bending the one piece of metal, I finally got a fairly decent connection that would allow me to use headphones as well as the speaker once they were disconnected.
It's not perfect, but it's not something I've ever read online as being a fix for a GBC with no speaker sound. I get tired of the knee-jerk reaction of people on forums just regurgitating things they've seen said, and have possibly been correct, before just to be the first to answer a question. But just because someone has said something is the answer, even with conviction, it's not always the right answer. Test the answer like a theory, don't accept it as fact and keep poking, testing and find the answer yourself.