Aureus Knights (AKS) has a very active Mumble community. We're a large, very active FC (consistently in the top 5 on the server), and quite a few of us hang out in our Mumble whether or not we're raiding together, so there's almost always someone chatting.

If you're interested, this is our thread on the Gilgamesh forum, this is our Lodestone page, and - most usefully - this is our website.