I found this cool tool to customize TTS voices on Garmins: changes phrases and/or levels/speeds, etc: (have not tried it yet on mine, but i think I am going increase Samantha's voice level a bit, and if that works, a few funny/different phrases):
http://turboccc.wikispaces.com/TTSVoiceEditor
http://turboccc.wikispaces.com/TTSVoiceEditor