Sorry, That option is for Mappoint ONLY.
igmon can do something like this with iguidance, but it's mostly centered around the 2.x series and hasn't yet been updated for 3.x
Have a look here for information on igmon:
iGMon - Solution for integration between iGuidance and Road Runner (I hope)
For those wanting volume changes for directions, if applications/scripts such as iGmon are able to detect when the directions are displayed, it's possible for them to lower the volume in RR and restore it after a few seconds. It only requires them to send simple commands to RR.