If a channel called "!demo" exists, it is used as demo mode, but also some global configuration. Primarily, this allows non-authenticated users to see all the web configuration pages as though they were a moderator; the system will pretend that everyone is a mod, but disallow saving. For actual changes to the demo mode's configs, use "Options | Localhost Mod Override", which will allow someone logged in as the bot's intrinsic user to be a real mod and make actual changes.
Whatever user the bot is authenticated with (Options | Change Twitch User or Options | Authenticate Manually) is the "intrinsic user". This user account is the one under which default authentication is done, and will be considered an admin (see eg Localhost Mod Override). This should be the bot owner.
On the reference deployment, this is Rosuav.
Any voice authenticated for use under the demo account will be listed on every channel's voice config page. They will not necessarily be available though. If logged in as the voice or as the intrinsic user, and if a mod for the channel in question (or using Localhost Mod Override), the voice can be activated for that channel.
On the reference deployment, this is Rosuav and MustardMine.
The demo channel's selected default voice, if any, is automatically available to all channels, and is the channel's default voice if none is selected. Each channel is still welcome to change its description but the voice cannot be deleted. It will be available in the command editor as "Bot's Own Voice" if it is not also the channel default.
On the reference deployment, this is MustardMine.