The Audio playthrough option will appear in Macintosh models which have an internal CD player which has the audio input connected to the logic board of the Macintosh via an internal audio cable. This connector is a four prong connector which essentially supplies the same capability as the positive and negative terminal on standard home stereo speakers right and left channel.
The computer will acknowledge this audio connector being present and provide the option for playthrough in the Sound Control panel. The PowerBook 500 series does not provide a connection for audio on it's logic board (the microphone connector does not count, see below). Because of this, it cannot monitor an audio signal from an external audio connector.
An audio connection made to the microphone port on the PowerBook 500 is unable to be monitored by the speakers of the PowerBook 500, until the signal is digitized and then played back through the speakers.
All Macintosh computers with internal CD support have the internal audio connection which will allow the playback of audio CDs through the internal speaker.
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