The small board near the hard drive, containing the 1/8" jacks also contains the audio chipset. It's a good sign the board and ribbon cable going back to the logic board are ok, since you get audio out from the headphone jack. Double-check the black cable going to the speakers, as that is what is probably loose.
http://www.ifixit.com/Guide/Repair/PowerBook-G4-Aluminum-17-Inch-1-1-67-GHz-Speakers/254/1#s950