Believe it or not, our new Toyota Highlander's entertainment system has such a feature, using the built-in microphone (It is a one-way "voice of God" thing, though).
Maybe some aftermarket head unit has it too?
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
Now!: 2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)