Forgot to check threading in Engine.play_tones()

darcs-hash:20090422134804-58103-283e2ee889c648ac2ddae46f873a746e3f06ed35
Play tones when receiving an incoming session while in another session

darcs-hash:20090422142249-58103-588cbb1f17c013305954fe4e42cf9f43364c88b6