What is the version of the Nokia Drive that you are using on the phone? You can check it by opening the application > options > about.
Things that you can try to sort out the issue
1) Uninstall the Nokia Drive, Tap and hold Nokia Drive from your Application list > select Uninstall
2) Go to Marketplace and download the latest version of Nokia Drive, after downloading the latest version of Nokia Drive make sure to download the voice guidance using a WLAN connection after successfully doing this try to check if the Voice guidance on your Nokia Drive is working again.
Hope this will help you.