1)
When is there song in the tents of Tzadikim?
1.
Rashi: This will be in the future. The song will be "Yemin Hashem Osah Chayil."
2.
Radak: It is when Hashem's right hand does for them Chayil (refer to 118:15:2:1).
3.
Malbim: When the king (Chizkiyah) walked from his house to the Beis ha'Mikdash, he heard it - "ha'Shir Yihyeh Lachem k'Leil Hiskadesh Chag" (Yeshayah 30:29), and Hashem made His voice heard. When they started singing the Shir of the Chag in Yerushalayim, Hashem began to fight Machaneh Ashur. The voice of salvation came together with the voice of song.
2)
What is the meaning of "Osah Chayil"?