I had exactly the same problem. The Sonic hdmuxer would not connect to sonic cinemaster 4.2 decoder and I could not build an avisynth script that worked with any other filters. Copying mfc71.dll to the system32 directory fixed the problem. I had to trust that this would work as I was very hesitant to copy an unknown dll to my system folder. Yet it did work. I just don't understand why. ANybody know why?