Two ways of doing this:
1) Go into Settings from the Nav screen (Cog on bottom right) Go to Guidance settings, toggle up to volume setting at the top and set to as low as possible. It isn't an easy operation though as is very hit and miss.
2) This one is easier when driving; as she gives an announcement rotate the volume on the centre console all to way back to silent. This will not effect your music volume provided you do this whilst she is speaking. However when done you cannot reinstate the voice on this method, has to be done using method 1 above.
To be honest the set-up to me is dreadful. One voice only, although she is polite. No mute operation on announcements by the touch of a button unlike TomTom which as simple to do speeding and camera warnings are disabled when her voice is turned down/off.
I wish you could turn the music off rather than simply mute, as by muting the sound the USB or CD continues to play silently in the background.
Live traffic - it isn't live or real-time, it is historical and invariably out of date plus setting your phone as a mobile hot-spot prior to your journey is a pain. Plus I wish I could disable that stupid warning that comes up!
Hope the above hints help.
Alex