Surround sound not being sent over the receiver/decoder when outputting 5.1 sound from the sound card's digital output in VEGAS.

Sound cards output digital audio in Stereo PCM or Dolby Digital/DTS encoded 5.1 audio only.  VEGAS products cannot encode in DD or DTS on the fly, which means they cannot send Surround signals from the sound card's digital output.

• If you have a sound card such as the Soundblaster Audigy connected to a Dolby Digital 5.1 receiver via a single digital cable, you will be unable to use 5.1 audio in VEGAS. Instead, you'll need to use a card with 6 discrete analog outputs (two stereo pairs and one pair for center/LFE). 
  Connect the card to the "DD ready" 6-channel input on the decoder/receiver. If you are using a 5.1 PC speaker system, you can also connect it using the three stereo inputs.
• To use cards such the Soundblaster Live! 5.1 and Extigy, you need to configure VEGAS to use the "Direct Sound Surround Mapper". This setting is located under "Options" > "Preferences" in the Audio or Audio Device tab.
• The Audigy and Audigy 2 cards come with ASIO drivers which also support audio output in 5.1 Surround sound. With these cards, you can use the ASIO selection or the Surround Mapper.
• Cards from other manufacturers work similarly. All digital outputs are limited to a single signal, which is encoded using Dolby Digital or DTS (in addition to Stereo PCM, which does not provide 6 channels). Check the manual for your sound card to see if the card has analog 5.1 outputs.