Updates have killed chat capability.

When the new Cataclysim (sp?) updates started some weeks ago, the ability to create and use custom chat channels stopped working. But the Party chat still worked when in a party. With the latest patches all chat channels, including Party chat, have simply stopped working. How to fix this?

There are 2 of us on XP, neither can get chat going.
One is on Vista and can't chat.
One is on Mac and can't chat either with the rest of us or with his other several playmates.

Yes chat is enabled.
Yes the right mic and headset are selected.
Yes the mic records and the recorded voice plays back.
Creating new chats, leaving chats, /resetchat, and all the usual "fixes" for nonfunctional chat don't work.
The problem does not appear to be realm specific although we've only tried two realms.