I had the same question on an SE Plus which has the Amundsen and steering wheel controls. Pressing in the left hand steering wheel thumbwheel does indeed activate phone voice control so isn't the solution to turning down the radio.
I couldn't work out for a long time how to turn the radio or CD player off and leave the satnav voice audible. I finally worked out that the volume control on the middle panel, and rotation of the left thumbwheel control on the steering wheel, do the same thing, and that either of them can mute (turn down to zero) the sound that is active at the time. So you can effectively mute the radio or CD, and then if you want the satnav voice back up just press the centre button on the Amundsen, to give you a current location, and while she is speaking use the sound level control to turn her up to the level you want. This leaves the radio muted.
Strange how hard it is to find this out. It would be so much more intuitive to have made the radio main button a 'toggle' so that it switches this on or off completely. Has anyone found a simpler way?