What voice calls out?
Rashi: Ru'ach ha'Kodesh calls out.
Radak: It is as if a voice calls.
Malbim: The verse depicts that he hears a voice calling, for the Navi sees the matter as if it already came.
Why does it say "ba'Midbar" and "ba'Aravah?
Malbim: Multitudes of wonders were done for our ancestors when they left Egypt, in a Midbar (desert). Just like a settled area has places of gardens and orchards, so a Midbar has a place of Aravah and pits, where thorns grow.
What is "Derech Hashem" that should be cleared?
Rashi: The road to Yerushalayim, for its exiles to return to it.
Malbim: The Navi sees with Ru'ach ha'Kodesh - just like then Hashem forged a miraculous path of Hashgachah, He prepared and guarded for their descendants to go in such a way at the time of the final salvation - "ki'Ymei Tzeisecha me'Eretz Egypt Er'enu Nifla'os" (Michah 7:15). He will add a new path - wonders that were never done before.
Why does it say "Panu" and "Yasheru"?
Radak: They will find in the Midbar and the Aravah a paved path, as if people cleared it for them.
Malbim: Panu is clearing rocks and obstacles from an existing path. Yasheru Mesilah is to make a new path.
Why does it say "lEi'lokeinu"?
Radak: He conducts this nation that leaves Galus.
Malbim: "Hashem" refers to the Creator, who renews the world. Elokeinu shows that He clings to Am Yisrael, and Hashgachah Pratis to do wonders for them.
