I understood exactly your setup before the detailed explanation.
My point was that it's not the best way to do it. Splitters will decrease your output volume, hence why i said to try connecting your MCE direct to your amp to check whether that works (without using the splitter). If it does, then either your splitter is decreasing the MCE output volume too much, or your splitter could be faulty. Also have you checked the volumes on your PC to make sure you stereo out isn't too low or perhaps muted? Is the cable plugged in to the correct output of your soundcard?
The ideal connection (assuming you have a surround sound amp - do you?) would be 3.5mm -> RCA on your TV, plus Coax/Optical -> AMP. This way there is no splitting as you have independent connections. This is how i have mine connected. I can listen in stereo on my TV (although it's usually muted), or anything up to 5.1 audio (depending on the source) through my 5.1 amp.