What is the meaning of "He'emid David Al Yedei Shir"?
Redak: He stationed them to sing. Malbim - he stationed those listed below (18-33).
Why does it say "Beis Hashem"?
Radak: This is like b'Veis Hashem. The same applies to "ha'Nimtza Beis Hashem" and many similar verses.
What is the meaning of "mi'Meno'ach ha'Aron"?
'Rashi': When they returned it from Beis Oved Edom, "va'Ya'azov Sham? l'Asaf ul'Echav", and it says "v'Oved Edom ben Yedisun v'Chosah l'Sho'arim."
Radak: It is from when the Aron rested in Yerushalayim when they brought it from Kiryas Ye'arim. It was in a tent that David made for it, until the Bayis was built. It says mi'Meno'ach, for until this day, it had no rest; they brought it from place to place. This day, David stationed singers, and they served in front of the Aron, like in the Beis ha'Mikdash.
