Xtrons Multimedia + GPS settings?

Hi good people,

I recently bought the 4.3" Xtrons all singing all dancing unit. Been very impressed with it, but cannot figure out how to set it so that when I am using the sat nav (GPS) part, I still get to hear the radio or MP3 audio.

The reverse normally happens, i.e. when I switch to radio or MP3, audio instructions from the GPS "break through" when necessary.

Any help will be much appreciated.