I think I've sussed my DOLBY D problem out.
Apart from the OUTPUT (to SPDIF) setting in ffdshow, I also had the DivX Tech Preview: MKV on Windows 7
software installed, which I took off when I removed the Dolby Surround software.
Today, I re-installed the DivX Tech Preview: MKV on Windows 7
software, as I couldn't get my MKV media files to play in WMC7, but as soon as I did, I lost the AC3/DTS signal on my receiver !!
Removing the software got me AC3/DTS back !!
Anyone know why this is, or if I need anything else to play MKV files in WMC7 please ??
I use the Windows7DSFilterTweaker
tool to set the preferred order in which the filters are selected on my PC, using COREAVC as the preferred H.264 codec for GPU use.