I've recently been poking around the Kindle's filesystem and came across the following directories:
The voice files contained within these directories are almost identical to voice files available as part of the Nuance RealSpeak demo and additional voices you can purchase here
The only difference is that the purchased files come with a Windows .dll, while the Kindle requires a Unix .so (e.g. /usr/lib/tts/speech/components/voicefont/vf_tom_full.so). I'm still looking for a source for alternate voices, but if there were, would anyone use them? I would create an installer that would replace the default voice on the Kindle with a different one.
All of this TTS investigation led me to start a write-up
on TTS for the Kindle; in Part 1, I interview Tom Glynn - the male voice of the Kindle (yes, I know Engadget interviewed him a year ago, but I had different questions).
I've also attached the interview file, so you can play it pack in Tom's "own" voice, which is kind of funny (to me at least).