If I understand you correctly: you are attempting to take the STEREO audio signal from your STB, feed it into your PC and then send if via optical SPDIF to your receiver in the hopes that when your receiver gets the signal, it will be a Dolby Digital signal?
Sorry, but that is not possible. You are feeding the computer a STEREO signal, so the computer will send your receiver a STEREO signal.
The only thing that you will achieve with such a setup is to change the medium by which the signal is transported. The contents of the signal will remain to be STEREO.
The only way for this to work (halfway) would be for you to process the STEREO signal with some software. However, doing that takes time (say 1 or 2 seconds). So, your audio would lag whatever video you are watching... I am sure this would be unacceptable so that is why I say it works halfway. I guess you could also feed the video into the PC and delay it (somehow). But you know what, this option is not worth the trouble... Just get an STB that works.
Sorry, but that is not possible. You are feeding the computer a STEREO signal, so the computer will send your receiver a STEREO signal.
The only thing that you will achieve with such a setup is to change the medium by which the signal is transported. The contents of the signal will remain to be STEREO.
The only way for this to work (halfway) would be for you to process the STEREO signal with some software. However, doing that takes time (say 1 or 2 seconds). So, your audio would lag whatever video you are watching... I am sure this would be unacceptable so that is why I say it works halfway. I guess you could also feed the video into the PC and delay it (somehow). But you know what, this option is not worth the trouble... Just get an STB that works.