Power MAC sound problem

[M A C]

My Power Mac G4 800Mhz with Built-in speaker
(not external speaker)
Use OS 9.1
I cannot hear sound from speaker(built-in) without plugin any device in the headphone or external speaker.

However after plugin the headphone, the built-in speaker have sound out. opposite headphone have not sound.

Can anybody have same problem and help me fix it ?
[img]images/smiles/icon_confused.gif[/img]