Bug 1109569 - Add sound notifications to the room paradigm in FxOS Hello

See bug https://bugzilla.mozilla.org/show_bug.cgi?id=1109569 please. Action happens at https://github.com/jaoo/firefoxos-loop-client/tree/1109569.
I would use `someoneJoined.wav` and `SOMEONE_JOINED_ROOM`, due to you are going to receive notifications only if you are the owner.