Albert Barnes 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)

The singers - No names are listed, and it is thought that they have been omitted from the text.

Were free - "Free," that is, from any special duties besides those of supervision. This supervision was arranged among the overseers so that one of them was on duty at all times, day and night.