The Gemara in Sanhedrin 94a states:
The Holy One, blessed be He, wished to appoint Chizkiyahu as the Messiah, and Sancheriv as Gog and Magog; whereupon the Attribute of Justice said before the Holy One, blessed be He: 'Master of the Universe! If You did not make David the Messiah, who uttered so many hymns and psalms before You, will You appoint Chizkiyahu as such, who did not recite song before You in spite of all these miracles which You wrought for him?'… Immediately the earth exclaimed: 'Master of the Universe! Let me utter song before You instead of this righteous man [Chizkiyahu], and make him the Messiah.' So it broke into song before Him, as it is written: "From the uttermost part of the earth have we heard songs, even glory to the righteous" (Yishayahu 24:27). A Tanna reported in the name of Rabbi Pappias: It was a reproach to Chezekia and his company that they uttered no song [to God] until the earth broke into song.
Why is singing a criteria to be Moshiach and why, in point of fact, did Chizkiyahu not sing?