Although you can work around this by either wiring it to aux in (satnav sound but nothing else) or wire up a seperate small speaker, I did this but with an aftermarket satnav, got a £4 speaker from maplin and fitted it out of the way behind the dash, was loud enough with music on to hear  
 
  
Not with Birdview I guess? 
  
I don't need the voice personally, they're rather annoying when they repeat themselves!!