Therefore - assurance of faith; or else the answer of Jehovah corresponding to their prayer. As surely as God redeemed Israel out of Egypt, He shall redeem them from Babylon, both the literal in the age following, and mystical in the last ages (Rev 18:20, Rev 18:21). There shall be a second exodus (Isa 11:11-16; Isa 27:12, Isa 27:13).
singing - image from the custom of singing on a journey when a caravan is passing along the extended plains in the East.