The speaker looked around, and then spoke with a thunderous voice.
Prepositions and prepositional phrases function in multiple ways in a sentence. They can be adverbs, adjectives, direct objects, or just plain prepositions. There are about 50 common prepositions. The word around is a preposition, but in this sentence it acts as a direct object as it receives the action. The only preposition in the word is with. So the answer is with.