Revelation 14:2

a voice from heaven like the voice of many waters
A singular voice, not the seven thunders (Rev. Rev. 10:4+), although elsewhere a (singular) voice describes a great multitude speaking in unison “as the sound of many waters” (Rev. Rev. 19:1+). This voice could be that of such a multitude contributing to the scene of worship. Or it could be the same voice which called the two witnesses up to heaven after their resurrection (Rev. Rev. 11:12+). Christ’s voice which John heard was also as “the sound, φωνὴ [phōnē] , ‘voice’ here] of many waters” (Rev. Rev. 1:15+ cf. Eze. Eze. 1:24; Eze. 43:2). Here it is probably the voice of the Father since the Lamb is said to be on Mount Zion. In some cases, the voice from heaven speaks down to earthly recipients (Rev. Rev. 11:12+; Rev. 18:4+) which would favor the view of the Lamb and the 144,000 being on the earthly Mount Zion. In other cases, the phrase merely indicates its originating authority as heard by John during the vision (Rev. Rev. 10:4+; Rev. 14:13+; Rev. 21:3+). like the voice of loud thunder
The sound John hears is like thunder, not actual thunder. Mighty angels have great voices which sound like that of a multitude (Dan. Dan. 10:6 cf. Rev. Rev. 19:6+) or the roar of a lion (Rev. Rev. 10:3+). A mighty voice, like that of the four living creatures (Rev. Rev. 6:1+). The participation of the voice from heaven indicates the significance of the scene. I heard the sound of harpists playing their harps
και φωνὴ ἤκουσα κιθαρῳδῶν [kai phōnē ēkousa kitharōdōn] , and [the] sound I heard of harpists. The MT and NU texts read somewhat differently: καὶ ἡ φωνὴ ἥν ἤκουσα ὡς κιθαρῳδῶν [kai hē phōnē hēn ēkousa hōs kitharōdōn] , and the voice which I heard as harpists harping. Does John hear a different sound from the preceding voice—the sound of harpists playing? Or is he merely describing further attributes of the voice—that the voice sounded like harpists playing? Adding to this complication is that φωνὴ [phōnē] can mean either “voice” or “sound.” Often, such subtle textual differences are of little significance, but here there is great significance to the variations. If John hears harpists playing their harps, then they which sing in the following verse could refer to the harpists (and not necessarily the 144,000 with the Lamb). On the other hand, if John is saying that the voice which he previously described sounds like harpists, then there may not actually be harpists. If the voice (singular) is in view rather than harpists (plural), then they (plural) which sing in the following verse probably refers to the 144,000. Another possibility is that the distinction between “voice” and “sound” in the English text (both translated from φωνὴ [phōnē] in the Greek) confuses the matter. Perhaps it is simply a sound which John hears. Thus, the MT and NU text could be translated:

And I heard a sound from heaven, like the sound of many waters and like the sound of loud thunder, and the sound which I heard was like harpists playing on their harps.

This would possibly describe a heavenly multitude which includes both singers and harps. If so, then they who sing in the next verse could refer to this multitude. See commentary on Revelation 14:1. harps
The four living creatures and twenty-four elders are said to have harps (Rev. Rev. 5:8+). So do those who have victory over the Beast, his image, and his mark—who are evidently martyred prior to the bowl judgments (Rev. Rev. 15:2+). Harps were often used in worship in the OT (2S. 2S. 6:5; 1Chr. 1Chr. 25:1-7; Ps. Ps. 33:2; Ps. 43:4; Ps. 57:8-9; Ps. 98:5; Ps. 147:7; Ps. 149:3; Ps. 150:3-6). Here, the harps are probably played by a multitude in heaven.