I tried both ffdshow_rev3342_20100331_clsid and ffdshow_rev3529_20100811_clsid, both from sourceforge. I used all default settings except enabling passthru of truehd and dtsma. I have 2 mkvs, one each with truehd and dtsma. I tried both in graphedit with mpc-hc mkv splitter and latest haali, and all kinds of audio renderers: nvidia wav and directsound, cyberlink 8 and 10, Arcsoft from TMT3, reclock with directsound, wave, and wasapi, and mpc-hc audio renderer.
I can get ffdshow to say that it is bitstreaming with some of those, but the receiver says no signal and I either get no sound at all, or static. My receiver is an Onkyo TX-SR705 and my vid card is EVGA 1GB 460GTX with the only version of driver available from Nvidia.
I really don't want to upgrade the machine to Win7. As silly as it sounds, one of the primary uses of the PC is the photos screen saver that comes with Vista. We use it to play our photos and videos all the time. The kids love seeing that on the wall in the family room. The windows live essentials version, the only one available for Win7, won't play full videos, only the first 10 seconds.