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

sounded 是动词 sound 的过去式，sound 在这里作系动词使用，意思是"听起来"。 relaxed 是形容词，来自动词 relax（放松），表示"放松的、自在的"状态。 sounded at his most relaxed 表示"听起来是他最放松的状态"。