Have you tried to reactivate the voice through preference then voice and reselect voice again. Also have you not accidentally turned the volume down? Have you touch bottom left of screen to access the volume?
Are you getting any sounds from the unit. It could possibly be the audio connection having gone faulty.
Another thing you could try is use another voice (or do a test on another voice to see if you can hear anything,
I would be very surprised if the voice files had become deleted by themselves. Connect the SatNav to your PC and physically check the voices directory to see if they are still there. If you have the voices on a memory card then you can just mount that through an adapter straight to the PC's usb port and use file manager to look for the files.