How to silence all but one application? (no calls, messages, notifications and similar interruptions)
If you don't want to use any 3rd-party apps, then you have to learn programming and make your own. 

In my opinion, the best way to do this is using AutomateIt.
Its normal features include "Set volume" actions and if you register an account, then you could download one Pro codec, like the "Multiple(OR)" event that, could also be used when more than one app are opened.

All types of volume can be changed when a certain event happens.