This is an option that's also important to me. I have a Lynx soundcard and use it as windows default (analogue outputs), but I use SPDIF from the motherboard in PowerDVD. WinDVD doesn't work this way and doesn't have a functional audio panel, so I had to stop using it when I got the Lynx soundcard.For me, an important feature would be to allow a different audio output than Windows' system default. I set Windows to use SPDIF so that I can use my receiver to decode and process everything. But I don't have HDMI on my receiver (or TV) so I use the analog outputs on my motherboard when I play blurays. PowerDVD allows me to set it at "6 channel" so that it uses the analog outs even though the system is set to use SPDIF. That's a big reason why I don't use TMT. TMT will only output sound if you set it as the same setting as Windows. If you choose 5.1 and Windows is set to SPDIF, there is no sound.
In the future I may be interested in using HDMI from a graphic card for movies, but stick to Lynx for music and other applications.