1. Check the volume by touching the speaker symbol on the main screen and ensure that it's not muted. If it is Mute will be ticked. Touch it to untick it.
2. If mute is not ticked and the volume is turned up touch tools on the main screen and then settings, then language and select a voice and then OK. If the voice file is not corrupted it should work then.
3. If the file is corrupted you'll have to go the garmin website and if you haven't already registered the satnav you'll have to do so. Download webupdater from http://www8.garmin.com/products/webupdater/howtoinstall.jsp
Follow the instructions on the website. Then click Next, Tick the voices you want on your satnav. Click Next and then Finish. Now you have the voices downloaded you can repeat the instructions in para 2 above to select the voice you want.