Did that on my S type, used the steering wheel mounted buttons to turn the volume down when the sat nav was "talking" and had to wait until it was talking again to turn it back up again !
Not sure about the S, but the XJ (in addition to satnav volume) has a voice guidance on/off setting in case that's of any use. Probably isn't that mind, since when voice guidance is off it doesn't mute the stereo.