Apple Performa 6400/180 - Manual (Page 29)

Basics Sound - 27
Sound Output
The 6400 and 6500 computers have one built-in
speaker/subwoofer and two sound output jacks, one on the
front and one on the back. Both output jacks are connected to
the sound amplifier. Inserting a plug into either jack
disconnects the internal speakers. The jack on the front is
intended for headphones, and the jack on the back is intended
for external speakers. The rear jack is muted when
headphones are plugged in the front jack.