Charles Ellicott Commentary 1 Chronicles 9:33

"And these are the singers, heads of fathers` [houses] of the Levites, [who dwelt] in the chambers [and were] free [from other service]; for they were employed in their work day and night." — 1 Chronicles 9:33 (ASV)

Refers to the singers discussed in 1 Chronicles 9:14–16: And these (above mentioned) are the minstrels, heads of Levitical families; in the Temple cells (they lived) , exempt from all other charge; for day and night they were over them in the work. The Hebrew is harsh, and perhaps corrupt, but the meaning seems to be clear. It is hardly meant that the service of song in the Temple was uninterrupted , but only that the choristers were under obligation to perpetually recurring service.

They were employed in that work. —Rather, They were over them in the work. They—that is, the leaders for the time being—lived, like the chief guards, in the Temple cells, presiding continually over the guilds of singers.