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This issue typically occurs on Dell Optiplex 5060 models utilizing analog audio for output. The other requirement is a (usually commercial) DVD which has 5.1-channel audio present.
Permanent fix (this will only keep the settings for your account on this device only)
- Open Preferences (Ctrl + P)
- In the bottom left corner where it says "show settings," click "All"
- Click "Audio" in the side bar
- On the right side of the pane where it says "Stereo audio output mode," select "Stereo"
- Click "Save" at the bottom of the window
Temporary fix
- Start playing disc in VLC (Ctrl + D, then "play")
- Select "Audio" from the menu bar
- Select "Stereo mode" from the Audio menu
- Select "Stereo." Audio should now be working.