Jamieson, Fausset and Brown on 2 Chronicles 35:15

15. And the singers ..., were in their place--While the priests and people were so much engaged, the choir was not idle. They had to sing certain Psalms, namely, the hundred thirteenth to the hundred eighteenth inclusive, once, twice, and even a third time, during the continuance of each company of offerers. As they could not leave their posts, for the singing was resumed as every fresh company entered, the Levites prepared for them also; for the various bands relieved each other in turn, and while the general choir was doing duty, a portion of the tuneful brethren, relieved for a time, partook of the viands that were brought them.