1)
Who will roar together?
1.
Radak #1: Bnei Bavel. This is like "Sha'agasi mi'Nahamas Libi" (Tehilim 38:9), "va'Yitechu cha'Mayim Sha'agasi" (Iyov 3:24).
2.
Radak #2: The troops of Madai who capture Bavel. This is like "Alav Yish'agu Chefirim" (2:15), "Sho'eg Likraso" (Shoftim 14:5), "ha'Yish'ag Arye ba'Ya'ar" (Amos 3:4) and similar verses.
3.
Malbim: That night they defeated Daryavesh, and Belshatzar made a feast for all his officers.
2)
What is "Na'aru"?
1.
Rashi: This is like 'Chamor No'er' (Brachos 3a; a donkey brays). Radak - it is like roaring, just Chazal used this word regarding donkeys.
