ITs kind of dark when you look at the back of the computer, and it isn't hard to plug the speakers into the wrong hole. I've done it several times.
Also my son walked accross the room and tripped over a speaker wire that I had hooked up to a repair--it never worked after that. Speakers can go bad.
If it takes batteries, they could be dead, or the kids are using them on something else without telling you
FYI: The speaker phones off of the cdrom are on a different hook up from the external speakers.