I use Mac OS X 10.8.4 and in Mumble I have set the audio input and audio output to "Default Device". Yet switching an audio input or output device in the system has no effect on Mumble. Mumble always sticks to the audio device that was set in the system when Mumble started running. Mumble never changes audio devices automatically when they are changed in the system settings. In order to change an audio device in Mumble I either have to set it manually or quit Mumble and start it again after selecting the audio device in the system.