Users can select different audio devices for different applications, so, for instance, it is possible to listen to music on the computer speakers while playing a game or having a conversation on the headset. However, this functionality is not very discoverable or accessible. At best the user has to go to the Volume controller in the Systray, go to Applications, and use the hamburger menu next to the application to select an audio device. I propose that it should be possible to find this hamburger menu (with a different icon) on the application title bars so users can quickly discover and switch between audio devices.
Audio can't be switched between specific windows, only apps. So I'm afraid putting this functionality in the window titlebars would be misleading. In addition, not all apps use titlebars, so the functionality would be missing in most GNOME apps, in Firefox and Chrome, and in all other apps using client-side decorations by default rather than traditional titlebars.