Hooray! I have tonight, finally got DTS MKVs working reliably in MediaPortal. It was actually a very simple solution in the end. I did a little experimentation around using the SPDIF output in my existing audio filter. I found that if I told it to decode to 5.1 it worked fine. So it was a problem with the audio filter passing audio straight onto the SPDIF output. Since, I’d used AC3Filter in the past I simply installed that, set it up for my amp (just checking which rates my amp would support via the SPDIF check utility provided with AC3Filter), and I was away. So, I now have glorious DTS sound again, the only difference from the last time I used AC3Filter on my previous Media Centre PC is that this one can play back 1080p video files easily.
Update (3rd August 2010)
Actually they don’t work! I thought they did, but only a few select ones did. Oddly my other media PC works perfectly. So, I’ve swapped them around and the fully working one is now in the lounge. It plays everything back without any problems at all. The plan is to clone that hard drive in order to get the other PC to a known working state.
Update (4th January 2011)
I have it all working now, but only because I had an enforced upgrade of my TV Server (due to a power outage/spike which partially fried some gear). So, with MediaPortal 1.1.2 and Stand Alone Filters v4 (with hardware acceleration enabled). I have 1080p DTS MKVs playing back just fine.