Frodo at Sammath Naur

[doug*platypus]

with a voice clearer and more powerful than Sam had ever heard him use

This doesn't state that a voice other than Frodo's was heard, so I don't think that this indicates the presence of another entity (Sauron or even the Ring) speaking through him. It is likely that the words, and the power with which they were spoken, came to Frodo's mind from an external source, but I believe that it was still Frodo's voice speaking.

Sam, Frodo, Merry and Pippin (!) are all at some point heard to speak with the words of another. But does this mean that they are not speaking the words themselves, that they have no will in the matter and are possessed, or acting as some kind of conduit? Possibly in Pippin's case this could be true, particularly given the description of the unnatural tone of his voice. What do you think? Was Frodo speaking with his own voice or not?

Personally I believe that Frodo had full knowledge of what he was doing. He made the decision himself (albeit under immense pressure from the evil of the Ring), spoke the words himself, and knowingly claimed the Ring for his own.