When using the headphone, audio playback cannot be heard.
There is a bug in the older Intel high definition audio driver that affects the microphone output. To fix the problem, update the kernel to version 2.6.12-9asl or higher. The precompiled kernel binary for Fedora Core 4 can be downloaded here.
http://updates.aslab.com/kernel/binary/FC4/i386
To update the kernel, download the three files and run the following command:
rpm -Uvh kernel*2.6.12-9*.rpm
If the laptop uses the Nvidia binary driver, edit /etc/inittab and change the run level from 5 to 3 before reboot the machine.
id:3:initdefault:
After reboot the laptop to run the new kernel, reinstall the Nvidia binary driver as documented here.