I won't speak of the "audible" part, since that's just a small matter of programming.
Turn by turn is... complicated. Of course, you can upload OSM maps on your Garmin right now and get turn by turn instructions. However, accuracy is a factor.
The amount of information needed to drive through a city is absolutely astounding, as is the frequency with which it changes. For example, a street near my house is closed mornings and evenings to vehicle traffic, except weekends and holidays and June through August. That data has to be in there to accurately route. "No left turn, 4-6 PM Monday through Friday." "No northbound traffic except bicycles." "Carpools only 7 AM to 10 AM"--God help us.
Not to mention just plain errors in the data. Near my house, an overpass was accidentally connected to the freeway. My Garmin with OSM data wanted to route me off the freeway directly onto the overpass. (I fixed the error.)
Realignments don't happen that often in cities any more in the US, but they happen on country roads and interstates *all the time*. I didn't realize until I started contributing to OSM exactly how much construction was always happening.
Highway 36 west of Red Bluff, CA, was recently realigned. Google even has it wrong for now: http://g.co/maps/mhdkm . And check this out: Google wants me to drive on a hiking trail: http://g.co/maps/jpxr8 I'm not saying they suck--Google's map quality is *exceptional*, and yet it errs. But I'd say that for turn-by-turn, it has OSM currently beat.
I guess what I'm saying is... uh, contribute to OSM. :-)