And I heard the voice of harpers (καὶ φωνὴν ἤκουσα κιθαρῳδῶν)
The correct reading is, καὶ ἡ φωνὴ ἣν ἤκουσα ὡς κιθαρῳδῶν and the voice which I heard (was) as (the voice) of harpers. Κιθαρῳδός is from κιθάρα a harp (see on Rev 5:8) and ᾠδός a singer. Properly, one who sings, accompanying himself on the harp.