1)
What is "Pitzchu"?
1.
Radak: It is raising the voice. This is like "Pitzchu Rinah" (14:7).
2)
Who will sing together?
1.
Malbim: When the lookouts (Yisrael) sing together with the heralds (Nevi'im - refer to 52:8:3:2), also the ruins of Yerushalayim will join them. Yerushalayim refers to the return of the masses and the gathering of exiles. This corresponds to "announcing salvation" (7).
