No more music-playin'!
In the "Concert of the Angels," some of our most devoted helpers are seen under God, in Heaven, with some instruments meant for merry-making.
Truth is, not all angels are 'messengers,' at least not in the sense that their sole purpose is to be a messenger from God to man. Some angels are 'helper' angels, and they help God at his throne. They probably spread a lot of joy through their singing, their merriment, and their love. We hear a little bit about these angels in Revelation 8: 2-5:
"And I saw that the seven angels who stood before God were given seven trumpets. Another angel came and stood at the altar, 3 holding a gold censer. He was given a great quantity of incense to offer, along with the prayers of all the holy ones, on the gold altar that was before the throne. The smoke of the incense along with the prayers of the holy ones went up before God from the hand of the angel."
While the painting above doesn't depict censers and trumpets, it depicts the fact that God rules over the Heavens and the earth (that big 'ol ball under his left hand), and he probably is pleased by good music!
More on the angels, from Catholic Encyclopedia.