Apple PC Compatibility Card - Manual (Page 98)

Connecting the Sound Out cable
15 Disconnect the audio ribbon cable from the Macintosh logic board.
This cable is connected near the DRAM DIMM slots. The connector has four
pins, and may be labeled “CD AUDIO” on the logic board.
Be sure to pull on the plug, not the cable. After you have disconnected the
cable, leave it in place for now.
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