Sounds like one of the contacts in the headphone jack are 'sprung.' One of the contacts in there touches another one when the headphones are
not plugged in. When they do not make contact, as when the headphones are plugged in, they cut out the built-in speakers (and the System assumes there is a headphone plugged in, of course). You may have accidentally pushed the spring-loaded contact slightly off center when you inserted or removed the headphone jack. Easiest 'fix' is to re-insert and remove that jack and see if things get back into alignment. If they don't, you
may be able to get access to the plug by disassembling the iBook case. More than likely, you'll be safer taking it in to the dealer...or just buy some external speakers, they'll have better sound than the very small ones in the iBook, anyway.