Yeah I had this issue the first time I 'properly' used the RNS510. Did trial runs with the satnav before and it was fine, but that's because I knew where I was going and the music wasn't too loud. But when I used it for real, missed a turn off because I couldn't hear the satnav voice over the music.
Dunno which unit you have but you should be able to:
- turn down the volume of the radio
- turn up the volume of the satnav voice (turn the volume up when the satnav is speaking; if not speaking you can press the button for the last instructions).
- in the menu, there's an option to determine how much the music is turned down when the satnav is speaking (just low/med/high options)