thread from the dead ... op was from January and first reply was from May
Hopefully the op will reply with the registry edit. I'd be interested, too.
The only thing I have been able to find regarding adjusting the volume of announcments is the following registry location:
HKCU\Control Panel\Sound Categories\Voice Command1
There are two keys:
AttenutationCategory
InitVol
credit: bengalih over at XDA
http://forum.xda-developers.com/showthread.php?t=391262