After I got my receiver, I wanted to pass through AC3/DTS to my receiver, so that it can decode the signals digitally. In VLC I had to select that option every time I play a movie. To default this,
first go to Tools >> Preferences >> Audio, and then under "Output Module", select "Win32 waveOut extension output." You should now see a check box that says "Use S/PDIF when available." Tick it, click "Save" and you should be good to go.