Right, I've had a look. If you turn down the volume whilst the satnav is speaking this is shown with a blue volume bar. If it changes back to radio, the volume bar is red.
The bottom button on the right hand side is the setup for the satnav. The second item on this menu is the volume. You can adjust this here and save your preferred volume. If you have it very quiet or right down at zero, the radio will still go quiet when it gives instructions,, even though you can't hear them.
So if you don't need spoken instructions, you can either turn off the satnav completely or just press "map" at the bottom left of the screen. There is no mute, though.