1)
Why does it say that Zekenim ceased from the gate?
1.
Ibn Ezra: They used to sit in the gates of Yerushalayim.
2.
Palgei Mayim: Old Leviyim used to guard the gates.
2)
What does it teach about Bachurim singing?
1.
Ri Kara: [Zekenim ceased from the gate, and] Bachurim ceased singing.
2.
Palgei Mayim: Young Leviyim used to sing [while Korbanos are offered. - PF]