Follow these steps to set up your voice:
1) Install free SVOX Classic Text-to-Speech Engine
2) Go to Android Settings -> Voice Input & Output -> Text-to-Speech settings, activate SvoxClassic at the bottom of the screen, then select SvoxClassic as the Default Engine.
3) Remove your previous version FIRST, then install and run this app.

Notes:
- Google Navigation requires that the Text-to-Speech voice matches the phone language. You can change the phone language in Android Settings -> Language & keyboard -> Select language
- If you receive error code 18 when trying to install a voice, the installation fails because of a locked file. Solutions are to mount the phone on your PC and manually remove "/mnt/secure/asec/smdl2tmp1.asec" or an empty folder named ".android_secure".
- On Android 2.1 and earlier, only some apps, like Google Translation, support changing voices.