Why did he stand them in Beis Hashem?
Malbim: This was like Chinuch (inauguration), for the Avodah had ceased for a long time. Therefore, he stationed Kohanim with trumpets, like the Chinuch of Shlomo and [afterwards] Bayis Sheni. He said "Mileisem Yedchem la'Shem" (31), for Chinuch is called Miluy Yad.
What Mitzvah did Hashem command via His Nevi'im?
'Rashi': The Leviyim will sing with instruments. The Torah did not [explicitly] say so. In Erchin (11a), we asked what is the source that Leviyim must sing with instruments; one answer was "La'avod Avodas Avodah" - Shirah with instruments is the Avodah that requires another Avodah. We can say that "Avodas Masa" (Bamidbar 4:47) refers to Shir. "U'Chananyah Sar ha'Leviyim b'Masa... Ki Mevin Hu" - he was appointed to raise the voices of Shir.
