Apple PowerBook (Duo, 2300 Series) - Manual (Page 58)

Connecting sound input and output devices
Your computer has a built-in microphone. For information on using the
microphone to record sounds, see the “Sound” topic area of Macintosh Guide,
available in the Guide (h) menu.
If you have a Duo Dock Plus or a Duo MiniDock, you can attach sound
devices, such as a tape player or headphones, to the dock’s sound input and
output ports. (You cannot use the internal microphone and an external sound
input device at the same time.) For instructions on selecting a sound device,
see the “Sound” topic area of Macintosh Guide, available in the Guide (h)
menu.
Microphone
49 Connecting Additional Equipment