After extensive testing with the new Construct2 update (that allows Universal Windows apps), it appears this plugin is somewhat broken. If I try to use the audio trick as per the tutorial (which was working before), it no longer plays audio. In fact, the plugin is no longer even needed for audio. Windows 8.1 and WP8.1 both work fine now with Construct2's regular audio object. It also works with the standard browser back functionality. So this actually solved most of my problems.
However, I have a need to detect if a user is on WP8.1 or not (to target a specific event). Unfortunately, the "Is Windows Phone app" condition is no longer working with WP8.1 either. Any plans to update this to work with WP8.1 Universal apps?
Update: The "is WP app" does actually work, but my condition is just ignored on my first loop for some reason. So I have one gap, followed by a seamless loop. It's an acceptable bug until WP or Construct find a better way to loop audio.