Download XGPS. Its a navigation app and free on Cydia or Rock and uses Google Maps but is a voice directed navigation. Probaly 1 of the best apps to date available. It is far superior than the plain google maps app that comes native with the Iphone. So this is your answer.
FYI - After I discovered XGPS I now hide my native google map app because absolutely will never use it again.
Mapquest Mobile offers a free turn-by-turn app; here's the iTunes link. It's one of the few (maybe the only?) apps out there that doesn't hit you up for a subscription after a free trial period--it's just free, forever.
Another jailbreak option is using Aloud, a tweak that will read your directions out loud--however, I don't believe it offers turn-by-turn directions as of yet, and it might require a newer firmware than you have.