There are so many issues with Garmin speech that I thought I could make a program to fix them. In the end, I stopped updating this program because I was getting many fix requests per week. I could not track all possible issues. Still, I hope some of the fixes provided by the TTS_Speech_Doctor are of some use to you.
You may also want to check the TTS_Speech_Doctor_Pro which allows you to make your own fixes. Have fun!