How to force VLC to use S/PDIF.

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.

The VideoLAN Forums • View topic - Help with a/52 over s/pdif

AC3filter Dolby AC3 passthrough

If you're sending AC3Filter's output to a surround sound equipped home theater receiver you only need to set the Output Format to AS IS (No Change) and PCM 16bit (for standard Dolby Digital or DTS) and check the Use SPDIF checkbox. This will bypass AC3Filter's internal processing and send the original signal through your S/PDIF connection.

More details here.