I've had this happen to me a few times now, and I am not 100% sure what causes this but my best guess is:
0. Have ATT on route guidance "ON"
1. Have music playing
2. Have a route active
3. When route guidance starts speaking, press next track.
From here on, route guidance voice is no longer heard. The only way I found to get it back was to restart CF.
Has anyone else seen this?