You can only have voice chat active in a single channel at a time.
Channel moderators and party/raid leaders can mute a player for everyone else in the channel.
Voice chat is not moderated by Blizzard. We recommend familiarizing yourself with the tools to keep annoying or disruptive voice chatters at bay.
A maximum of five people can be heard simultaneously through voice chat.
Since the voice chat servers are separate from the game realms, you can still chat while zoning, or even while alt-tabbed out of the game if you have the "Sound in Background" option enabled in the in-game sound options.
When a party converts to a raid you'll automatically be moved to the raid channel.
Voice chat will display the nameplate for who is talking when you are in a custom channel.