Currently Being ModeratedMay 8, 2013 7:28 AM (in response to Gofa)
This can be done on cellular data enabled iOS 6 devices. To change the Navigation Voice Volume in Apple Maps, follow the information in this article under "Settings for Maps":
Settings for Maps
To modify your settings in Maps, tap Settings > Maps:
Note: The Navigation Voice Volume setting is available only on cellular-enabled iOS 6 devices.
For additional information you can also see the user guide here (Maps starts on page 75):