Could you speak up? We can't hear at the back.

speak 是动词，意思是"说话、讲话"。 up 在这里是副词，表示"向上、提高"，与动词搭配时表示动作的增强或加大。 speak up 是一个短语动词，意思是"大声说话"或"提高音量说话"，常用于请求别人说话声音大一些。