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I spoke too soon.
I have the 5300 and thought it would have similar options.
The SR5000 does not have an Auto selection.

So if I understand the issue correctly:
You have your PC hooked up to...

First question back should have been what is your source?

Your audio field selection should be "Auto".
This will allow the received to detect DTS 5.1/Dolby Digitial 5.1/Stereo
When you select...