And I heard a voice from heaven,

This may be redundant as we have already established in the previous verse that this is taking place in heaven. Perhaps the voice in coming from another part of heaven.

*  like the sound of MANY WATERS and
*  like the sound of LOUD THUNDER,

On earth the audio spectrum is a very limited spectrum. Perhaps this is John's way of saying the frequency range is expanded on both ends.  That is, the MANY WATERS expands the high frequencies and the LOUD THUNDER expands the low frequencies.

Likewise on earth, sound requires a substance to travel through like air, water, or other solid material. Now if there is no substance in heaven which is likely since it is spiritual in nature, then this voice is communicating the song (next verse) by a totally unknown mechanism. This method of communications also is described in Revelation 1:15, Revelation 19:6, and Daniel 10:6.

and the voice which I heard was like the sound of harpists playing on their harps.

God is a HAPPY God, and harps are instruments of JOY. When the LAMB shows up, a song breaks out (Psalms 40:2-3 and Ephesians 5:18-19)

Harps express the emotions one is going through. On the bright side, Psalms 33:1-3 and 1 Samuel 16:14-23 show a lifting emotion. On the cloudy side, Job 30:31 and Isaiah 24:1-8-13 show the silence of a harp is like 'mourning'.