One of the 613 Mitzvot, is the commandment that the Leviim serve in the Beit Hamikdash. Their primary role was to physically guard the actual building, as well as singing to accompany the Tamid sacrifice, morning and evening. There was a rotation organized where specific Leviim had their day to sing. Those who could not sing, because they had bad voices, served by doing guard duty.
The idea was to give honor to G-d, and the Place, with a known tribe, who were placed their permanently. A Levi could be disqualified from serving if he did not personally observe all the Mitzvot of the Torah. There always needed to be a minimum of twelve Leviim to sing at any given time. The instruments played in the Temple were not done exclusively by the Leviim. Their main task was to sing. Those who played instruments had to make sure that they did not drown out the singing.
The Torah spoke of the Leviim working between thirty and fifty. This was strictly related to the Mishkan and carrying it, and dismantling it. They could work in the Temple, as long as they still had a good voice. He could still do guard duty even if their voices were gone.
Nowadays, we view all Leviim as having the status of being acceptable and kosher Leviim. Today, they are given the honor of receiving the second Aliya to the Torah. They wash the hands of the Kohanim before the Priestly blessing. Their first born males are exempt from the Mitzva of Pidyon Haben. In the desert, the Leviim had to shave all the hair on their bodies. Over the years, I have met numerous Leviim who are bald, but sing beautifully. They are true Leviim, and I think all Leviim are very pleased that they are Leviim!