"Shim'u Amim" is second person. "Kulam" is third person!
Radak: Verses often switch between second person and third person, e.g. "v'Ulam Kulam Tashuvu u'Vo'u Na" (Iyov 17:10), "l'Chulam ul'Sarneichem" (Shmuel I, 6:4). Also Michayhu 1 ben Yimla said "Shim'u Amim Kulam" (Melachim I, 22:28).
Michah is not Michayhu, who prophesized earlier, in the days of Yehoshafat, like it says there. (PF)
Which "Amim" does he address?
Radak, Malbim: He addresses Yisrael; "Amim" (plural) refers to the Shevatim, like "Amim Har Yikra'u" (Devarim 33:19).
What is "Eretz u'Melo'ah"?
Radak: Eretz Yisrael and the nation in it.
Malbim: It is the Klal of the land; the evil will spread to the entire land.
Why does he tell the land to hearken?
Radak: This is a metaphor.
About what will Hashem be a witness?
Rashi: That I prophesized to you in His name, and warned you.
Malbim: He warns you.
What is "me'Heichal Kodsho"?
Radak: It is from Shamayim.
Malbim: He warns you while He is in His Heichal, before he leaves to judge the land.
