[Raynor]

Doesn't it say somewhere that the Palantiri did not work through the medium of sound anyway?

Indeed, in Unfinished Tales:

Originally Posted by The Palantiri

Alone the palantiri could only "see:" they did not transmit sound.

I am suspicious that they in fact worked in some ways via sanwe

I agree - at least this is what we make of Pippin and Sauron's discussion:

Originally Posted by The Palantir

He did not speak so that I could hear words. He just looked, and I understood.

(although Pippin is sort of lipsynch-ing- probably due to lack of skill.

I am not sure that Denethor was unknowing of Sauron; he did fight against him and his pull towards Mordor, with great expense of energy (although aided by the fact that he inherited authority).