Currently, there is nothing forcing you to set an @voice for your character, and hardly any notice that you should.
@voice is the command that you use to change the message that comes up while your speaking, describing your characters voice. It's a nice flavor thing when you're in a conversation with someone. It's also the biggest identifier of you, as a character, when you're conversing with someone over the phone.
It's really nice to be able to identify your ace-cool on the phone because you know his voice message so well. It's fun being able to track down your stalker who calls you up at midnight by identifying his voice in a bar. It feels almost abusive, in my opinion, when someone says something to you and there is no way to identify who it might have been because they were an @voiceless person saying one generic thing.
Extra credit for those who invest in disguise, or artistry, or something in order to change your voice to trick people!