Many applications are designed to mute their audio when they are not the foreground window.  In general this is a good thing as you do not want angry birds playing music while you switch to something else,  but in other cases like netflix it would be nice it if did not.

Music playing apps and the youtube player on the other hand are designed to continue their audio even when they are not the main app.

We will be looking into offering an option to override the apps choice, but on a per app basis.