Could you clarify for me what you are doing here? You have a STB with optical output and taking this into a Sound Blaster device. You can then "listen" to this via Windows, then using WASAPI loopback in MC to pass it through the audio engine, but only by selecting a different output device in MC from what is being fed into the Sound Blaster?
I've been trying to work out how to feed external audio through MC without an ASIO driver. What you're doing sounds indirect but would get round that problem.
Next question is how to set this up so that it can work from MC by simply changing a zone or something! Presumably that's not possible?
What I did:
1. Set the digital out from my integrated sound card (Realtek) to be the default output device in sound properties
2. set the Digital In (SB Zx) as the default recording device
3. opened Digital In (SB Zx) properties and selected listen sending it to the Realtek Digital out
4. in MC click file-->Open Live-->WASAPI Loopback
If you are using a Creative product I suggest not having their control panel running as it seems to cause problems with this at times.
I have created a Zone (Optical In) and I just keep the loopback there in the playlist so all I have to do is switch to it and I have sound from my STB
going through the MC engine. I have noticed some slight lip sync issues that I have not tried to remove or at least reduce yet.