And I heard a voice from heaven, as the voice of many waters, and as the voice of a great thunder. Evidently the song of the heavenly inhabitants, as described also in Revelation 7:9-11, where we are told they "cried with a loud voice."
The greatness of the voice is evidence of the vastness of the number. "Heaven," from which the sounds come, includes the "Mount Zion" of Revelation 7:1, on which the Lamb and his followers stand. And I heard the voice of harpers harping with their harps.
The Revised Version is better, and the voice which I heard [was] as [the voice] of harpers harping with their harps. This reading is supported by א, A, B, C, and other good authorities. As the voice; that is, in regard to its pleasantness; reminding the hearer of the temple worship.
(On the word "harp," see on Revelation 5:8.