I'm just using YouTube as a convenient external source. Mostly I only use the WDM for browser sourced audio, but other sources work the same, unless they are sending unusual audio formats, like Tidal with MQA Master Audio.
For the success case that I mentioned, when I closed MC GUI, the video froze solid.
When you say "closed the GUI", do you mean minimised, or completely shut down?
For the WDM Driver to work either the MC GUI or Media Server or both must be running, as it is MC that is processing the audio.
So, there are a few options here:
1. You could set MC and/or Media Server to start running with Windows. "Options > Startup >Windows Startup > Run on Windows startup" set to "Media Center", "Media Server", or "Media Center and Media Server"
2. You could just start the Media Server when you wanted to use the WDM Driver. Main menu in the GUI > Tools > Advanced Tools > Media Server".
3. You could just minimise Media Center when you are playing an external source, and optionally have MC minimise to the System Tray. "Options > General > Interface > Minimise to System Tray"
I have never seen the video lock up no matter what I do to MC when playing via the WDM. I just tested some more:
1. MC running without Media Server. Start YouTube video. Close MC completely. Sound disappears. Restart MC GUI. Sound restarts about 5 seconds later. Very robust.
2. MC running with Media Server. Start YouTube video. Close MC completely. Sound disappears for about 4 seconds, then restarts. Restart MC GUI, sound continues without a glitch, with the visual display of audio in MC in sync. Close Media Server, leaving MC running. Sound continues without a glitch. Couldn't be more robust.
3. MC running without Media Server. Start YouTube video. Minimise MC with "Minimise to System Tray" set. Sound continues without a glitch. Maximise MC. The same result, as expected.
This was all on my Workstation, which is a simple PC with a motherboard based Realtek soundcard, and just a couple of speakers. But it is a fair test of the WDM Driver itself.
What browser are you using? I am using Google Chrome. I just tried 1. above in Internet Explorer. It took 10 seconds for the sound to return. The video kept playing no problem. Microsoft Edge took 8 seconds for the audio to start again.
Windows Media Player playing music; 8 seconds for audio to restart. Playing video, 10 seconds to restart audio, with some minor pausing when MC was restarted, but no lockup and video kept playing when MC was closed.
VLC and DivX video player, 5 seconds to restart. No video glitches.
Only the free version of Zoom Player gave me any issues with audio in video playback. But then I couldn't just select the Windows Default audio device in that. It works more like MC in audio device management, selecting the device directly.
So what is different about what you are doing, or your system?