Several users reported that their voice stopped working in Google Navigation. The reason was a mismatch between the voice you’ve purchased and the default language on your Android. Update 2.0.6 of the Classic engine fixes this problem. The UK English voice will be used even if the language setting on your device is US English, and vice versa.
Alternatively if you use the UK English voice you can go to Android Settings -> Language & keyboard -> Select language and select English (United Kingdom) .