Windows 7 - onboard sound card, but I am using a usb adapter on one of my audio sources (Logitech G330 USB adapter)
Mumble 1.2.3 RC2 (and earlier)
Mumble seems to only check the default audio device on startup, and when explicitly told to do so in the settings (ie open up audio devices and hit apply). Switching default audio device on Windows 7 does not result in an audio device change in Mumble. Audio continues to play through the previous default device.
Set Mumble to use default audio device in the Audio Input pane.
Go to Control Panel -> Sound -> Playback devices, and set a different playback device as default.
Check to see whether sound is being played out of new audio device.