Tomtom 510 not talking?

Tomtom 510.I set up my tomtom and everything comes up as required accept it does not give audio directions. I have tried the reset button and still nothing happens.I have gone into the set up directions and tried to alter the volume and press test and nothing however sometimes it starts to talk but in a minuite or so it stops again. Any ideas please or do i need to take it to a repairer?