According to William Smith in his Smith’s Bible Dictionary, the harp was the national instrument of the Hebrews, and was well known throughout Asia. He reports Josephus, the Jewish historian, as stating that the harp (usually) had ten strings and was most times played with the fingers. That was why David the prophet said we should praise God “upon an instrument of ten strings, and upon the psaltery; upon the harp with a solemn sound” and “Praise the LORD with harp … and an instrument of ten strings.”. - Psalm 92:2, 3; Psalm 33: 2.
The harp had a distinct sound, (1 Corinthians 14:7) and, in the hands of a skilful player like David, was so melodious that it soothed the nerves. David played for Saul, when he was possessed by an evil spirit and this calmed him down. (1 Samuel 16:16, 23) In time of old the harp was used to make music at religious and social occasions. (Genesis 31:27; Job 21:12; 1 Samuel 10:5; 1 Chronicles 25:3) The captivating power of the word of God is likened to the hold which delightful music has on one, hence David says “I will incline mine ear to a parable: I will open my dark saying upon the harp.” - Psalm 49:4. David was a composer of songs and was inspired to write many deep sayings, and prophecies in the psalms. The book of psalms therefore contain the inspired words of God arranged in such a way that they could be rendered as songs in the original Hebrew language.
The harp was used symbolically in the vision which John the apostle saw in the island of Patmos, regarding the number of those of the saints class and the role and rewards of the apostles. He stated, “And I looked, and, lo, a Lamb stood on the mount Sion, and with him an hundred forty and four thousand, having his Father's name written in their foreheads. And I heard a voice from heaven, as the voice of many waters, and as the voice of a great thunder: and I heard the voice of harpers harping with their harps: And they sung as it were a new song before the throne, and before the four beasts, and the elders: and no man could learn that song but the hundred and forty and four thousand, which were redeemed from the earth.” - Revelation 14:1-3.
The lamb is Jesus Christ. (John 1:29; 1 Corinthians 5:7) Mount Zion is the capital part of God’s Kingdom in heaven. (Psalm 48:2; 132:13,14) The 144,000 constitutes the complete number of those of the apostles’ class of whom the 12 apostles were the foundation members. Hence it is stated that they “were redeemed from the earth” and “were redeemed from among men, being the firstfruits unto God and to the Lamb.” (Revelation 14:3, 4) The apostles’ or little Flock class are raised at the second presence of Christ to be immortal spirit beings who will also be kings in God’s Kingdom. They are the ones whom Christ told, “Ye are they which have continued with me in my temptations. And I appoint unto you a kingdom, as my Father hath appointed unto me; That ye may eat and drink at my table in my kingdom, and sit on thrones judging the twelve tribes of Israel.” - Luke 22:28-30. See also Luke 12:32.
That they had the Fathers name written in their foreheads means that they had the accurate knowledge of God, the truth by virtue of the anointing spirit. The forehead is the seat of intelligence. Compare Revelation 3:12; 7:3. The music coming from such a multitude would of course produce a booming sound that would be reverberating through the heavens, hence it is said to sound like the sound of many waters (many people), even like thunder. “No man could sing that song” means that no body can understand the truth and dispense it to the people if he is not anointed. This calls to mind the statement in the book of Ezekiel that because what he was preaching was like melodious music to the people, they loved to hear it. The problem however was that they were not prepared to do what they had been told. “And, lo, thou art unto them as a very lovely song of one that hath a pleasant voice, and can play well on an instrument: for they hear thy words, but they do them not.”- Ezekiel 33:32.
The ten symbolic strings on the harp of God refer to the ten fundamental doctrines of the Holy Bible which are revealed only to the saints. The ten fundamental truths are Creation, Justice Manifested, Abrahamic Promise, The Birth Of Christ, Ransom Sacrifice, Resurrection, Mystery Revealed, Our Lord’s Return, The Glorification of the Church, and Restoration.
That the servants of God are taught by God Himself through the holy spirit is symbolized in the books of Ezekiel and Revelation by the eating of a scroll which is sweet in the mouth but bitter in the belly. (Ezekiel 2:9,10; 3:1-3; Revelation 10:8-10) Apostle John stated: “But the anointing which ye have received of him abideth in you, and ye need not that any man teach you: but as the same anointing teacheth you of all things, and is truth, and is no lie, and even as it hath taught you, ye shall abide in him.” - 1 John 2:27. See also Galatians 1:11-17; John 14:26; 15:26, 27; 16:13,14; etc.