The "arm" in the poem refers to

An arm refers to a branch of a big tree.

"Listen, the Voice of Autumn" is the text of Lesson 7, Unit 2, Volume 1, Grade 3 compiled by the Chinese Department of Primary School of People's Education Press. The author Bi and Zi Jun participated in the recording.

This text is a modern poem. The author praised autumn in poetic language by grasping some sounds of nature in autumn.