Possible to play app sound when minimized?

At least for the app I often use, Ximalaya FM, I am sure I can keep the app sound playing while using another app.
For the app you mentioned, it is not clear whether this is possible. If not, I think you should contact the developer for confirmation.

 

I know media apps, like music and podcast players, those can play their sound in the background while using other apps. And I wouldn't want a media app that couldn't. However games usually don't play sounds when in background.AFAICT. I'm sure it's something that's set by the developers, when they make their apps. In the case of RuneScape, that would be something the Jagex devs would need to do. Maybe worth you emailing them about it.