What will Tziyon hear?
Radak: The sound of the thunder.
Malbim: The conduct mentioned here is via nature. Tziyon and Yerushalayim have a more elevated conduct, which is miraculous with personal Hashgachah; it overrides nature. Tziyon will hear that Hashem is overseeing all nations to conduct the Ma'arachah based on Tzedek and Mishpat.
Why will Tziyon and Yehudah rejoice?
Radak: They know that the hail and fire descend to punish the Resha'im who gathered against Yerushalayim for war. It mentions Tziyon, for it is the capitol of Yisrael's kingdom, and Resha'im will be punished nearby, in Emek Yehoshafat. Bnos Yehudah are the cities of Yehudah.
What is "Mishpatecha"?
Rashi: It is Your vengeance.
