The Lord laid down strict instructions as to what musical instruments were to be employed in public worship in the Temple. These instructions relate to four musical instruments, namely: the trumpet, harp, psaltery and cymbals.
The use of the trumpet was present from the very beginning of the Mosaic economy. The silver trumpets were to be blown at set times, cf. Numbers 10:2-.
It was not until the time of David that the other three instruments, the harp, psaltery and cymbals, were added to the Old Testament form of worship, cf. 1 Chronicles 25:1,6: Moreover David and the captains of the host separated to the service of the sons of Asaph, and of Heman, and of Jeduthun, who should prophesy with harps, with psalteries, and with cymbals: and the number of the workmen according to their service was … All these were under the hands of their father for song in the house of the LORD, with cymbals, psalteries, and harps, for the service of the house of God, according to the king’s order to Asaph, Jeduthun, and Heman. These were the only four instruments permitted!
The instrumentalists are said to 'prophesy' in using these instruments, cf 1 Chronicles 25:1,3. They were ministering to the congregation. We have an example of all four of these instruments being employed when the ark of the covenant was brought up to Jerusalem, cf. 1 Chronicles 15:16,28; 16:5,6. This was a very joyous occasion. The ark had resided outside Jerusalem until then. David brings it to the place where God had said back in the days of Moses that He would put His name.
During times of revival when there was a reforming of worship these four instruments were again put back in their rightful place in Temple worship. Examples are found during the days of Hezekiah, Ezra and Nehemiah, cf. 2 Chronicles 29:25-27; Ezra 3:10; Nehemiah 12:27.
On the part of Hezekiah, Ezra and Nehemiah there was a going back to the old ways when only these four instruments were to be employed at the command of God. Therefore, it is firmly established which four instruments were permitted in Old Testament temple worship, ie. the trumpet, harp, psaltery and the cymbals.
However, there are other instruments mentioned in the Scriptures which were not to be employed in Temple worship but were still to be used in the praise of the Lord on other occasions. For example:
 Nine times we read of the timbrel or tabret, same Hebrew word both places, it was a tambourine type instrument;
 Ten times we read of the pipe or flute; also the cornet is mentioned ten times which is an instrument closely associated with the trumpet;
 The organ, singular or plural. This was another wind instrument, something like panpipes today.
Interestingly, there are commands, some in the Book of Psalms, to employ these other instruments in the praise of the Lord. Yet as already observed they were commanded in the Old Testament to employ only the trumpet, harp, psaltery and cymbals in temple worship. Examples are:
 The Timbrel or Tabret, cf. Psalm 68:25 The singers went before, the players on instruments followed after; among them were the damsels playing with timbrels.
Psalm 81:2 Take a psalm, and bring hither the timbrel, the pleasant harp with the psaltery.
Psalm 149:3 Let them praise his name in the dance: let them sing praises unto him with the timbrel and harp.
Psalm 150:4 Praise him with the timbrel and dance: praise him with stringed instruments and organs.
 The Pipes, cf. 1 Kings 1:40 And all the people came up after him, and the people piped with pipes, and rejoiced with great joy, so that the earth rent with the sound of them. This was at the coronation of Solomon.
 The Organs, cf. Psalm 150:4 Praise him with the timbrel and dance: praise him with stringed instruments and organs.
How can there be commands to use only four instruments, ie. the trumpet, harp, psaltery and the cymbals; yet we also find commands to, and examples of, use of other instruments, such as, Timbrel/Tabret, Pipes and Organs on occasions when God is worshipped?
The explanation is found in making a distinction between the direct religious worship in the Temple in Old Testament times and what were essentially national celebrations. In direct religious worship in the Temple only the four instruments mentioned were to be used. On national celebrations these other instruments were permitted to be used.
If we do not maintain this distinction then we have to accept that God contradicts Himself in His Word and that cannot be the case.
However this begs the question: how could these Psalms, which contain commands to praise the Lord with these forbidden instruments, be suitable to be sung in the public worship in the Temple in Old Testament times? The worshipper would be singing exhortations forbidden by God for worship in the Temple. The exclusive psalmist today makes great play of singing inspired praise as this is the safeguard against unsuitable expressions that are not glorifying to God. However, to argue that all the psalms of the psalter were sung in the Temple, even those which contain these commands to sing with instruments forbidden is accusing the Old Testament worshipper of doing this very thing.
To reconcile these issues it must be accepted that not all the psalms were evidently sung during worship in the Temple and that some of them were employed only on national occasions when a lesser standard applied.
Therefore, do exclusive psalmists today believe in singing all these psalms with these commands to employ instruments not permitted in Old Testament times?
Previous posts on this subject:
1. The terms 'Psalms, Hymns and Spiritual Songs' inEphesians 5:19 & Colossians 3:16 do not refer to the 150 Psalms of the Book of Psalms.
2. Old Testament and New Testament saints did not live by this exclusive psalmist rule.
3. The angels and glorified saints did not sing the psalms.
4. There are possible remnants of hymns/canticles/doxologies quoted in the New Testament.
5. Progressive Revelation argues against Exclusive Psalmody.
6. Exclusive Psalmody leads to different levels of worship.
7. The Exclusive Psalmist's position requires them to reject ever singing the 'very best song' in public worship.
8. If 'inspired praise' is required then an equally valid argument could be made for 'inspired praying' and 'inspired preaching' in public worship.
9. The early New Testament Church did not believe in Exclusive Psalmody.
10. The Reformers at Geneva did not believe or practice Exclusive Psalmody.
11. The Puritans did not all believe in Exclusive Psalmody.
12. The Westminster Divines did not believe in Exclusive Psalmody.
13. Scottish Presbyterianism has not always believed in Exclusive Psalmody.
14. The metrical translation of the psalms is not an accurate translation.
15. Singing Psalms which make mention of musical instruments.