1)
Why should they sing with harps, trumpets and a Shofar?
1.
Malbim: They sing with harps to tell His praises and constant deeds. They sing with trumpets and a Shofar, for whenever they sing in front of Him, He reveals Himself in His honor to them; Shofar and Teru'ah show this.
2)
What do we learn from "Lifnei ha'Melech Hashem"?
1.
Radak: They will come to the Beis ha'Mikdash to blow Teru'ah, sing and praise in front of Him.
