John Gill 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)

And these are the singers, chief of the fathers of the
Levites
Others of them were employed in singing, and were masters of the song, and presided in that service, see (1 Chronicles 6:31) who remaining

in the chambers were free ;
from all other work and service; and there they abode, even in the chambers of the temple, that they might be near to perform their work, and not be defiled, and made unfit for it:

for they were employed in that work day and night ;
some or other of them, either in composing or singing psalms and hymns, or teaching others how to sing them.