Is there a command to change the audio output device in xbmc under the systems screen.
Not that I know of. I have tried. But there is a way to bring up the audio settings screen while watchIng video. You just need the correct window id.
Update: Now possible in Gotham. see "Setting audio outputs and passthrough in XBMC Gotham": http://voxcommando.com/forum/index.php?topic=1583.0