System Tray Audio Device Switcher uses "SoftwareMicrosoftMultimediaSound Mapper", "Playback"
to set the index of the sound device which was obtained by enumeration the devices.
mciSendCommand from "winmm.dll" is also used
In this source code you will find the registry keys used to achieve that.
If this doesn't work you could give Process Monitor a try and monitor all registry activities of windows when you change the default device. On my Vista installation the control panel twiddles with "HKLMSOFTWAREMicrosoftWindowsCurrentVersionMMDevicesAudioRender"
For Vista see
http://www.vistaaudiochanger.com/
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…