Revelation 19:1

I heard a loud voice of a great multitude in heaven
John heard a similar voice (or noise) from heaven when the 144,000 redeemed of Israel learned the new song (Rev. Rev. 14:2-3+). There, the voice (or noise) was not explicitly identified as a multitude, but mention was made of numerous harps so it is probably a similar heavenly multitude in both scenes. The voices are of all those in heaven including: (1) the voices of the Church, having been raptured before the Tribulation, (2) the voices of the Tribulation saints, many of whom were martyred (Rev. Rev. 6:9+; Rev. 7:14+; Rev. 12:11+; Rev. 13:7+, Rev. 13:15+; Rev. 15:2+; Rev. 16:6+; Rev. 20:4+), and (3) the voices of various ranks of angels. They sing in unison as one mighty, thunderous voice (Rev. Rev. 19:6+).

Alleluia!
A transliteration of the Hebrew, הַלְלוּ־יָהּ [hallû–yāh] , a command: You all praise Jah! (e.g., Ps. Ps. 106:48).² The first occurrence of the word Hallelujah in the OT corresponds with the context of its usage here: “Sinners shall be consumed out of the earth, and the wicked shall be no more. Bless Jehovah, O my soul. Hallelujah!” (Ps. Ps. 104:35, Darby’s Translation).³

Salvation and glory and honor and power
The great multitude, who came out of the Great Tribulation, attributed salvation to both God and the Lamb (Rev. Rev. 7:9-10+). Salvation from God is said to have come at the casting out of Satan, the accuser of the brethren, from heaven (Rev. Rev. 12:10+). Glory and honor and power are attributed to God by the twenty-four elders because of His act of creation (Rev. Rev. 4:11+). Every creature in heaven and on earth gives glory and honor and power to both God the Father and the Lamb (Rev. Rev. 5:13+). Their praise recognizes the salvation found in the redemption provided by the Lamb: “You are worthy to take the scroll, and to open its seals; for You were slain and have redeemed us to God by Your blood” (Rev. Rev. 5:9+) and “Worthy is the Lamb who was slain” (Rev. Rev. 5:12+). Here, salvation is probably broader than personal redemption, but includes the rescue of all of creation, and especially the earth, from the corrupting influence of the Harlot (Rev. Rev. 19:2+). See Worship of God.

Notes

¹ The equivalent phrase, הַלְלוּ־יָהּ [hallû–yāh] , is found in the OT.

² Piel imperative, masculine plural. An intensive command. יָהּ [yāh] is short for יָהוְה [yāhw^eh] (cf. Ps. Ps. 68:4).

³ “In the Hebrew text, it appears at the ends of Psalms Ps. 104:1, Ps. 105:1, Ps. 115:1, Ps. 116:1, Ps. 117:1, at the beginning of Psalms Ps. 111:1 and Ps. 112:1, and at the beginning and end of Psalms Ps. 106:1, Ps. 113:1, Ps. 135:1, and Ps. 146:1.”—Robert L. Thomas, Revelation 8-22 (Chicago, IL: Moody Press, 1995), Rev. 19:1.