The Talmud recounts that King David's harp would play by itself at midnight.
And the Jerusalem Talmud comments that in the verse, "When the musician would play music, the hand of God would be upon him," the word for "musician" should be read as "musical instrument" instead: the instrument would play by itself.
Why do our sages insist on conflating musician and musical instrument?
The answer is that when a person attains the perfect level of prayer, he himself is an instrument playing music spontaneously.
He has become the instrument of song, of prophecy, of prayer.
The melody is his, and his entire being is none other than an instrument expressing that prayer.
Such self-expression–in which a person links himself not only to an inner state of holiness but also to the external structures of holiness–is prayer on the highest level.
Although such an achievement is exceedingly rare, we must make it our goal.
We must know that such a level exists.
Even if such a state is not common, it can sometimes spark into flame from time to time, in a psalm, a blessing, or even a word.
–Rabbi Adin Steinsaltz
From "Prayer" in The Thirteen Petalled Rose (new edition) by Rabbi Adin Steinsaltz