Sorry, That option is for Mappoint ONLY.
In RRConfig, there is an option to reduce sound to some percentage when saying directions. Three questions: Does this work with iGuidance 3.0? Does this mute the audio like MapMonkey does? If so, which audio line does it mute?
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.