1 Chronicles 9:33

Overview - 1 Chronicles 9
The original of Israel and Judah's genealogies.
The Israelites;
10 the priests;
14 and the Levites, with Nethinims, which dwelt in Jerusalem.
27 The charge of certain Levites.
35 The stock of Saul and Jonathan.
Treasury of Scripture Knowledge

1 Chronicles 9:33  (King James Version)
And these are the singers, chief of the fathers of the Levites, who remaining in the chambers were free: for they were employed in that work day and night.

the singers
6:31-33 15:16-22 16:4-6 25:1-31 Ezra 7:24

were free
Nehemiah 11:17 Nehemiah 11:22 Nehemiah 11:23

they, etc
Hebrew upon them. employed.
A number of Levites were employed by rotation in singing the praises of Jehovah; and they seem to have continued the service day and night: see the References.
Psalms 134:1 Psalms 134:2 ; 135:1-3