Had some good fun making my own voice file using the VoiceStudio download from Garmin for my 1340. Of course, you don't have to stick to the script. Mine is much less annoying now that it says "DOOHHHH!! Recalculating!" doing my best Homer Simpson impression. In fact it's worth taking a wrong turning to see the look on your passenger's face!
Just one problem - it doesn't speak street names like the official 'Emily' or 'Daniel' and it can't find Mo's Bar.
Make the sound file (mp3 or wav) and name it the same as per the POI file it goes with.
ie. speed_camera.gpx file
speed_camera.wav or mp3
then put it in the same directory where the POI files are and load them with POI Loader.