At the press conference, he sounded at his most relaxed.

sounded 是动词 sound 的过去式，sound 在这里是系动词，表示"听起来"。 relaxed 是形容词，来自动词 relax（放松），表示"放松的、轻松的"。at his most relaxed 表示"处于最放松的状态"。