www.pdafun.net/ is the official site for Google Nav.
I only have the trial version myself. Reroute and voice commands are part of the full feature package. The site appears to be poorly translated Japanese, and even some of the screenshots show Japanese characters on their phones.
I read on the site that the downloaded voice files need to be unzipped into the SOUNDS directory on your phone after you install the software, but I checked my trial version and the sounds English sounds are already preinstalled. Worth checking yours though:
program files\google navigator\sounds\100.wav, etc... (see if they open and play in media player when you double click them maybe...mine do)
Mo