Why will Shamayim sing?
Radak: This is a metaphor. The entire world 1 will rejoice when Yisrael leaves Galus.
Malbim: It will be seen to all that made Shamayim and conducts the Ma'arachah. People will not err to say that Hashem abandoned the land to the rule of the stars.
Radak: The verse mentions the sphere (Shamayim), the central point [of the land], inanimate and vegetative, but not the living, for not all living will rejoice; Bavel will mourn.
What did Hashem do?
Radak: He did good to His nation, and redeemed them.
Malbim: Refer to 44:23:1:2.
What are "Tachtiyos Aretz"?
Radak: It is the central point of the land. Malbim - also there, Hashem's honor and Hashgachah will be seen.
What is "Pitzchu"?
Rashi: It is an expression of opening the mouth.
Radak: It is raising the voice.
Why does it mention the mountains?
Radak: The primary creation of the land is the mountains in it.
Malbim: The Hashgachah will not be general. Rather, there will be individual Hashgachah on every mountain, forest, tree and vegetation.
Why does it say "Yispa'er"?
Rashi: Kel's glory is taking His nation out of Galus. The nations will glorify Him for this. So Koresh said - "Kol Mamalachos ha'Aretz Nasan Li Hashem
