This is how I understand the technical side: Metro is originally designed to "disconnect" apps from continuing audio/video when switched to the background; there's a Technet (?) paper around that describes this default behavior as a user-friendly decision to prevent people from getting confused if something starts playing in the background and they have a bunch of apps running. However, it is possible for app developers to program their apps to continue playing audio (described
here, though a bit out of date for 8.1).
Under normal circumstances, if an app isn't programmed to actually do this, the default behavior naturally has precedence. The only way around it is to snap the app.
But in this case, D1vad says it continues to play for him, so... Check for app updates?