I have a C510, and the unit has stopped producing the text-to-speech function. When on route, it will say turn left, when before it used to (and is meant to) say turn left on hilltop road. I have checked all the settings, and selected restore on screen where you select the option you want to change, with no luck.
I have just purchased a C510 ( sent from the UK ) and it does not have text to speech, apparently only the Aussie units have TTS, I have seen posts on other forums about hacking into the UK/US versions to get TTS working and the posts indicate that the hack works with software V5.4 but not V5.5, have you updated from V5.4 to V5.5? maybe this is why your unit stopped using TTS.