Here is how to mute the Navigation voice during a call on an Android phone. Basically, you will download an App, have it notice whenever your "Phone" app is running (this is the case whenever you receive a phone call), and have it mute the Media Volume during the call. Here are the steps:
1) Download Tasker from the Android Market. It will set you back a few bucks.
2) Open Tasker, and create a new profile. Don't name it. Click checkmark to save.
3) The "First Context" window will pop up. Choose "Application", and then scroll down and select "Phone." Save.
4) Click "New Task". Don't name it, Save.
5) Click the "+" to add a new action, select "Audio", and select "Media Volume".
6) Change the Media Volume to zero.
7) Save everything. You're done now. The obnoxious navigation voice won't talk over your conversations anymore. $6.50 well spent, congratulations.