Apple PowerBook (2400c) - Manual (Page 61)

Connecting sound input and output devices
Your computer has a built-in microphone. In addition, you can connect an
Apple Omni microphone to the external sound input port.
For information on using the microphone to record sounds, see the “Sound”
topic area of Mac OS Guide, available in the Guide (h) menu.
Your Macintosh PowerBook has a stereo audio input port (marked with the
√ icon) for connecting external sound input devices that provide line-level
output. You cannot use the internal microphone and an external sound input
device, such as an Apple Omni microphone, at the same time. For instructions
on selecting a sound device, see the “Sound” topic area of Mac OS Guide,
available in the Guide (h) menu.
IMPORTANT Although the internal microphone is deactivated when an
external sound input device is in use, it’s possible to record sound from more
than one sound device at the same time. This can occur because the computer
merges sound from devices attached to the SCSI port or PC Card slots. If two
or more devices produce sound, the sound from all of the devices may be
recorded when you use the SimpleSound program or the Monitors & Sound
control panel to record. To eliminate the sounds you don’t want to record,
remove or turn off the device producing those sounds.
Microphone
61 Using PC Cards and Additional Equipment