Ok, there's nothing you can do to get 5.1 from an stereo input source. The Phillip's DVD 5.1 Surround is done internally of the player so from the computer using the Aux input will only be stereo. Having Windows set to 5.1/7.1, it separates each channel apart for it's designated speakers, that's why some of your sounds are missing.
If you want proper 5.1 surround you're going to have to need a new AVR (AV Receiver) with Optical or HDMI connections that can decode multi-channel surround, or just change Windows to stereo. all AVR's today will have these connections that can be used by a sound or graphics card from the computer that have HDMI or Optical.
Something like;
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