Halakhah do Liczb 4:49

But the core of their service is to sing over the sacrifice. And they would only say song with communal burnt-offerings that are obligatory and upon the peace-offerings of [Shavouot] at the time of the wine libation; but not over voluntary offerings that the community would make for the 'end of the altar.' And so [too], they would not say song upon libations that came on their own. A Levite who is bereaved (onen) is permitted to serve and sing. We do not ever decrease below twelve Levites that stand on the platform to say song over the sacrifice, but we [may] increase [as much as is wanted]. And the song that the Levites would say was with the mouth, as the crux of song is with the mouth. And others would stand there that would play musical instruments - some of them Levites and some of them pedigreed Israelites that [their family be permitted to] be married to [priests] - as only the pedigreed are ever able to go up to the platform. And we never reduce below nine lyres, but we [may] increase [as much as is wanted]. And there was only one cymbals. And it appears that the reason is that the sound of cymbals is loud and a bit frightening; and [so] if there were many, the rest of the musical instruments that were there would not be heard - all the more so, the song of the mouth. And the hollow of the flutes that they would play with were of reed, since their sound is pleasant. And the flute would strike [play] in front of the altar twelve days a year. And it pushes off the Shabbat, since it is [a part of] the service of the sacrifice, and the sacrifice pushes off the Shabbat. And a [Levite] does not come to the service until they teach him for five years. and as they, may their memory be blessed, learned from the verse. And from here, they, may their memory be blessed, said (Chullin 24a) that a student that does not see a good sign of (some success) for five years, will never see [it]. And he does not ever come to serve until he is an adult, as it is stated (Numbers 4:49), "each man to his service." But a Levite is not disqualified by age and not by blemishes, but rather by deterioration of the voice, such as the old men whose voices deteriorate at the time of old age. And that which is stated in the Torah (Numbers 8:25), "But at the age of fifty they shall retire from the legion of service" is only at the time they were carrying the tabernacle from place to place. And even at the time of old age when his voice deteriorates, he is not disqualified from guarding the Temple and from closing the doors. And all of this and the rest of its details are nicely elucidated in [various] places in Tamid and Middot; also some of this matter is elucidated in the second chapter of Arakhin. And I have written a little at length about this for you, my son, as the Temple will soon be built, and you will need it. Amen, so should God do (see Mishneh Torah, Laws of Vessels of the Sanctuary and Those who Serve Therein 9).