I had the same problem. Here is your answer straight from the Apple website. MAC-simus is right about the microphone.
From the Apple Support page:
PowerBook G4 audio line in port requires a powered signal
The PowerBook G4 audio line in port takes signals from audio equipment, including line-level devices such as amplified or self-powered microphones, and is specifically designed for high-level audio signal such as the signal from a powered microphone.
If you use a nonpowered microphone with the audio line in port, it will result in a very low volume signal, or possibly no signal at all.
Third-party options, such as Griffin Technology's iMic, allow the use of many mic-level sound input devices with your computer.