(n.) One who speaks for another.

adviser agent announcer annunciator authority champion channel communicant communicator connection debater declaimer delegate demagogue deputy enlightener expert witness front front man go-between gossipmonger grapevine haranguer herald informant information center information medium informer interagent interceder intercessor intermediary intermediate intermediate agent intermediator intermedium internuncio intervener interventionist interventor interviewee jawsmith mediator medium middleman monitor mouth mouthpiece negotiant negotiator negotiatress negotiatrix newsmonger notifier official spokesman ombudsman panelist press prolocutor prolocutress prolocutrix prophet protagonist public relations officer public speaker publisher rabble-rouser radio ranter rapporteur reporter representative source speaker speecher speechifier speechmaker spieler spokesperson spokeswoman spouter talker television teller tipster tout tub-thumper voice witness