Actually stop null sound device on ConferenceBridge

darcs-hash:20090703103354-58103-6e227ff399a3ec8f4604d229a2cdd8ec53731e3c
Minor fixes in checks of sound components in core

darcs-hash:20090703103438-58103-c5df5fd9fbf6d2d171f3bfd64702ed0e677deca1