General:
* Orator (formal, eloquent speaker)
* Lecturer (someone who gives formal talks)
* Presenter (someone who presents information)
* Talker (someone who speaks a lot)
* Voice (a person speaking)
* Announcer (someone who announces something)
Specific to a device:
* Loudspeaker
* Microphone
* Transmitter
More formal:
* Prolocutor (person who speaks on behalf of a group)
* Spokesperson (person who represents a company, etc.)
* Advocate (someone who speaks for a cause)
More informal:
* Chatterbox
* Mouthpiece (someone who speaks for another)
Figurative:
* Mouth (the organ of speech)
* Voice (an expression of opinion)
The best synonym for "speaker" will depend on the specific context.