Greg Giffords of Digital Dealer said recently that we don't necessarily need to change the way we (as car dealers) are optimizing our sites for voice search. I think his reasoning was that many of the voice searches (at least for now) are telling Siri to set reminders, alarms, get phone numbers to places, etc..
I'd bet that an extremely small amount of voice searches are even related to dealerships or automotive in any way (other than something like "What is Buerkle Honda's phone number?") and that our energy could be better spent elsewhere. I think that in the current state of voice search and our industry, focusing your energy on other things will be more effective. Unless you're a 30-store group with F-U money and you'd like to give it a go.
However, I still think its important to pay attention to voice search so that you can prepare for the future. Maybe we'll see an upward swing in its usage or its relevance to our industry and it will make sense to allocate resources to it.