The voice files for the camera warnings are OGG files which match the file name of the associated ov2/ bmp files for the POI. To re-enable these warnings go to Change Preferences/ Manage POI/ Warn When Near POI and select the first speed camera POI in the list, configure the options and when the unit displays "Is this the sound you want to hear" (even though it plays nothing at all) select Yes, if you press No the device will crash.
Repeat the setup for each of the remaining camera categories in the same way as above.
A quick tip for the distance to use when setting the options, use the "times ten rule" if its a 30MPH camera set it to 300 yards, 70MPH set it to 700 yards, I am sure you can work out the others in between

this equates to between 25 to 30 seconds prior to the camera that you get the warning assuming you are travelling at (or close to) the rated speed for the road, for mobile camera I add 50% to the above figures - Mike