Luckily for you, I have another program that allows you to add some missing voice. Try RGNVoiceEditor. Latest version seems to be v109. Find it here:
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1- It modifies RGN firmware only, but you could use RGN_Tool to convert it back to GCD (Remember that GCD firware will not update if same version as current GPS version)
2- It should work correctly with voices like Aussie English and some others. For foreign languages, it is possible that all voice prompts will be mixed up.
3- You should not have to convert VPM voice to Taiwan voice (button in RGNVoiceEditor) unless you have a TWN firmware. The patch changes the order of some navigation prompts.
4- Replace TTS with TTS and non-TTS by non-TTS voices.
5- No worry if the program shows only the first 15 voices.
Let me know which fw and gps model you are using.
*** Use at your own risk! Although I think it is pretty safe now! ***
Note that I have never seen a non-TTS Australian_English. Only TTS Karen and Lee.