They sing as it were a new song (αιδουσιν ως ωιδην καινην). See 5:9 for this phrase (cognate accusative) save that here ως (as if)…

And they sang, as it were, a new song. (See Barnes on Revelation 5:9.)

It was proper to call this "new," because it was on…

And they . . .—Translate, And they (i.e., the heavenly harpers) sing a new song (the words “as it were”…

And I heard a voice from heaven, as the voice of many waters, and as the voice of a great thunder: and I heard the voice of harpers harping wit…

This “new song” should be related to the “new song” in 5:9, also sung by the angelic choirs. It is the song of redemption and vindication. What was…

And they sung as it were a new song
The song of redeeming grace; the same with that in (Revelation 5:9) …

Mount Sion is the gospel church. Christ is with his church and in the midst of her in all her troubles; therefore, she is not consumed. His presenc…