Peter is representing the twelve apostles as their spokesman. Though we can assume that the other apostles were also speaking to people (Acts 2:4, Acts 2:11), the time has now come to speak with one voice to all those who have gathered. As a result, Peter stands up so that he can be heard by the crowd.1 He likely spoke in Aramaic, the one language that would have been understood by all the Jews (also those who came from abroad for Pentecost).2
14 But Peter, standing with the eleven, lifted up his voice and addressed them: “Men of Judea and all who dwell in Jerusalem, let this be known to you, and give ear to my words.