Apple PowerBook (2400c) - Manual (Page 62)

The Macintosh PowerBook has a stereo sound output port (on the left side of
the computer’s case, marked with the - icon) to which you can connect
externally powered speakers, an amplifier, headphones, or other sound output
devices. The sound output port accepts a standard stereo miniplug, like that
used to attach headphones to a portable tape player.
Your computer has a speaker in the front of the case, which plays sound from
an external stereo device connected to the computer through the sound input
port, or from a zoom video card in the lower PC Card slot.
You can adjust sound output quality in the Monitors & Sound control panel.
(For instructions, see the “Sound” topic area of Mac OS Guide.) The
computer automatically selects the sound output source. If an external device,
such as headphones, is connected, sound is routed there; if no external output
device is connected, sound plays through the computer’s speaker.
Audio in port
Audio out port
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