I just got my Nuvi 760 back from repair (only one week away, well they sent me a new unit not repaired mine) and I was pleasantly surprised to see that there is a small upgrade in that the new unit has two microphones now, one on the left and one on the right hand side now.
I'm pretty sure that they have installed 2 mics because Garmin sells its devices to the left-wheel drive countries and to the right-wheel drive countries as well. With 2 mics on both sides the quality of sound for your interlocutor does not depend on the country where you're located.