What is "Sadai"?
Radak: It is as if it says Sadeh, i.e. the field and Midbar.
Why will they rejoice?
'Rashi' (Divrei ha'Yamim I, 16:32): He returned the Aron to us, and sent damaging angels to do Mishpat to the Pelishtim.
Malbim: Even though Shamayim does not change, and it gives its rain normally, the land is excited according to deeds. If naturally, it is proper that there be little rain, even so the field and all in it will rejoice, for the rain will fall only on the fields, and they will produce grain. This is another way that Shamayim has Simchah and the land has Gilah. Also refer to 96:12:4:2.
What is "v'Chol Asher Bo"?
Radak: It is beasts of the field.
Will the trees truly sing?
Rashi: This is a metaphor for all rulers of the nations.
Malbim (based on Rosh Hashanah 17b): Sometimes people are liable, and naturally, it is proper that there be much rain, the rain will fall only on the forests, and not on the grain fields, and all forest trees will rejoice.
