Music is sent to all speakers. Navigation is only sent to the front and override music when played.
Volume set for either is important though.
The volumes for music and navigation are independent.
It depends on the volume you set either as to if you hear one, the other or both.
Example 1: Both have roughly same volume
You will hear both navigation and music. When navigation cuts in, you'll still hear that music is playing in back.
Example 2: Music is set high, navigation normal
Navigation will cut in, but it might sound faint. The music in back is clearly overpowering.
Example 3: Music is set high, navigation low
Navigation will cut in, but will almost be silent. The symptom will be that music only plays in back for a period.
Solution:
If you change the music volume, make sure you change the navigation volume to suit.
To change music volume, turn up/down when music is playing.
To change navigation volume, easiest way is to wait for navigation to speak then use volume knob/switch - OR, hit the NAV button, wait for the voice "you have reached your destination" and change volume at this point. Also volume can be controlled on map, but it's more fiddly when driving.
I don't often set routes, but I always have speed camera alerts set to ON. So I need to hear the GPS.
(Although quite lucky in this state anyway, because we have three warning signs prior to a fixed speed camera. But audible warning is always great).