I don't want to speak for the staff or admins here, but as I don't think anything has changed...
Mumble is a very minor part of Famcraft. Not to say it isn't important or anything, but it's absolutely optional and not even necessarily recommended. I'm pretty sure there's usually someone moderating it, but you wouldn't normally find too many people on it at once.
In-game voice chat makes up about 98% of chat, I'd say. Every now and then, a player or staff member won't be able to get onto Minecraft but can still talk through the chat using IRC, and Mumble isn't even connected to the in-game chat at all.
Your son shouldn't feel pressured to use it -- often the only time players would use it (or the only common reason I can think of) is if they feel really comfortable with another player and often play or build together on Famcraft, and prefer talking orally than chatting (naturally it's easier to communicate).
Hope that clears it up a bit, I'm sure the admins can help a bit better.