Orthodox Christian worship is best executed by the instrument for which it was developed - the human voice. Perhaps more to the point is the fact that the Apostles and early churches did not use instruments in worship, and indeed emphasized this "absence" as being significant.
This is why we need to pray with simplicity and humility. We need to be 'selfless' in our prayer. Just as a nightingale sings without anyone seeing, we need to give ourselves over to the worship of God in secret.