Why did Hashem make the ends of the land hear?
Malbim: All the dispersed from the four corners of the world should gather. The Navi always depicts the gathering of exiles before the building of Yerushalayim - "Eleh me'Rachok Yavo'u;
What reward is discussed?
Rashi: What is prepared to give to His servants.
Radak: It is for what they suffered in Galus for the sake of His great name.
What is "Pe'ulaso Lefanav"?
Rashi: The reward for the work that they did for Him.
Radak: It is the good act that they did in Galus - maintaining His Torah and Mitzvos with the great afflictions. The suffix Vov refers to Kel - they did for His sake. The same applies to "v'Simachtim b'Veis Tefilasi" (56:7; it is called Hashem's, for it is for His sake). "Sechar" applies also to this. "Lefanav" is with Him.
Radak citing Targum Yonasan: The reward for doing His word is with Him; and their deeds are revealed in front of Him.
Malbim: In addition to paying Yisrael, Hashem desires to do this act that He previously promised.
