I just had the same problem and finally got around to fixint it today. I took an old PSP 1000 speaker, desoldered the old speaker in the Game Boy, and soldered the PSP speaker in, like
this. After i had gotten the speaker installed i accidentally hooked up the power backwards, causing a small diode to burst into smoke, but removing it and creating a solder bridge fixed that. Good thing nintendo put that there!
I would have replaced it but i didn't have any diodes that size rolling around.
edit: Sorry about the image size, its huge for only being 5MP o.O
edit: I couldn't get spoiler tags to work so i made a gallery that you can go to.