What is "Vela'amod lifnei ha'Eidah" referring to?
Rashi: It refers to singing on the Duchan.
What are the connotations of the word "Vela'amod lifnei ha'Eidah Lesharsam"?
Seforno: It implies that the Levi'im serve Hashem with Shir and the Mishkan by carrying and the like (guarding), and this service was done in front of the congregation to stress their inferiority, inasmuch as the congregation was disqualified and the tribe of Levi were chosen to replace them. 1