Who are "Tzofayich"?
Rashi: They are lookouts stationed on the walls and towers, to herald and see who came to the city.
Radak: They are your Nevi'im. A Navi is called Tzofeh - "Tzofeh Nesaticha l'Veis Yisrael" (Yechezkel 3:17), because he is Tzofeh (sees) future events. After, it explains what will be their voice.
Radak citing Targum Yonasan: Your leaders.
Malbim: They are Yisrael.
What is the meaning of "Nas'u Kol"?
Radak: They will announce loudly, amidst Simchah.
Malbim: The verse depicts that at the moment that the herald announces, the lookout sees it overtly, and he shouts that he saw it.
Why will they sing together?
Radak: They will see clearly Hashem's return to Tziyon. I.e. they will see a prophetic vision that the honor returned to Tziyon, i.e. Ru'ach Nevu'ah, which ceased when Chagai, Zecharyah and Malachi died. At the time of salvation, it will return, and even greater than it was.
Malbim: The Nevi'im who herald will sing with Yisrael who look. Refer to 52:8:2:2.
Why does it say "Ayin b'Ayin Yir'u"?
Radak: This is like "Ayin b'Ayin Nir'ah Atah Hashem" (Bamidbar 14:14). Everyone had Ru'ach Nevu'ah at Ma'amad Har Sinai, aside from overtly seeing the honor.
Malbim: Until now, Nevi'im saw Hashem's return to Tziyon in prophetic visions, but not with a physical eye. Now they will see with a physical eye (kingship, Kehunah and the Beis ha'Mikdash, which are of Tziyon). This corresponds to "heralding good" (7).
