I think neal can configure one of his Music Vaults that way.
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If you just want to listen to HDTV audio in another room maybe the easiest thing is to get wireless speakers.
In order to generate digital stream from analog stereo source, you'll need hardware device called ADC (analog to digital converter) to digitize the audio. There are a couple of problems. The first problem is the latency. ADC will add fair amount of latency, probably in 100's of ms or probably more. The audio will have noticeable delay from the video, and you'll have the lipsync issue. Secondly, the HDTV audio is in digital format. It'll be converted to analog from HDTV, then to digital using ADC, then back to analog from Roku. I wouldn't go down that path.