WiFi Ad-Hoc is usually used to connect two computers together to form a temporary network without any infrastructure like routers and wireless access points.
Getting it to work with multiple stations and without manual interaction is not really possible without extensive hacking and re-writing of code.
What you are looking to do has already been done and there are already specifications and standards for this.
It's called an InVANET - Intelligent Vehicular Ad-hoc NETwork - and is designed specifically to allow any vehicles within range to talk to each other without any manual intervention.
The communication specification is 802.11p which is not the same as WiFi which is 802.11a/b/g/n (n is still only draft).
Once you have the network in place transmitting voice can be done in many ways. The network, if done properly, will be an IP network, so anything that can transmit across the Internet can transmit across the InVANET - including multicast messages (see mbone for example).