I did some testing and managed to get the speaker dock to work. Initially, it wasn't working but I played a video coming through the tablet speakers, removed the tablet from the dock while still playing the video, reattached the tablet to the dock and the audio automatically switched from the tablet to the dock.
Exiting the apps, restarting them while docked, and the audio continues to work through the dock.
If I restart the tablet while docked, playing a video has the sound coming from the tablet. Removing and redocking, gets the audio to come from the dock.