Acer Aspire 4320 User Manual page 54

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The Playback Devices dialog box will pop up. There are three tabs: Playback,
Recording and Sounds. Click the Playback tab, and then select Realtek Digital
Output and click Set Default to set the digital output.
For enjoying movies, games and music in a realistic, multi-channel
audio environment, please see Acer eAudio Management.