Asus M3A78-EH - Manual (Page 46)

1-34 Chapter 1: Product introduction
7. Digital audio connector (4-1 pin SPDIF_OUT)
This connector is for an additional Sony/Philips Digital Interface (S/PDIF)
port(s). Connect an HDMI module cable to this connector, then install the
module to a slot opening at the back of the system chassis.
Ensure the audio device of Sound playback is Realtek High Definition Audio
(the name may be different based on the OS). Go to Start > Control Panel >
Sounds and Audio Devices > Sound Playback to configure the setting.
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M3A78-EH Digital Audio Connector
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