Angels are referred to together as a heavenly host

Psalm 103:20, 21 Praise the LORD, you his angels, you mighty ones who do his bidding, who obey his word. Praise the LORD, all his heavenly hosts, you his servants who do his will.

Psalm 148:2  Praise him, all his angels; praise him, all his heavenly hosts.

The idea of hosts is also attached to armies and to stars. The angels form a great army, an army of light. (Cf Job 38:7; Revelation 9:1, 11).

Angels are sometimes referred to as Sons of God

Job 1:6 One day the angels came to present themselves before the LORD, and Satan also came with them.

Job 2:1 On another day the angels came to present themselves before the LORD, and Satan also came with them to present himself before him.

Job 38:6, 7 On what were its footings set, or who laid its cornerstone - while the morning stars sang together and all the angels shouted for joy?

(Psalm 89:6 For who in the skies above can compare with the LORD? Who is like the LORD among the heavenly beings? [Sons of God])

Not only Christ an his people but angels, because they are so close to God, are known in the Bible as sons of God.

Angels are holy but they lack the perfection found in God

Job 14:17, 18  If God places no trust in his servants, if he charges his angels with error, how much more those who live in houses of clay, whose foundations are in the dust, who are crushed more readily than a moth! 

Job 15:15, 16 If God places no trust in his holy ones, if even the heavens are not pure in his eyes, how much less mortals, who are vile and corrupt, who drink up evil like water! 

These parallel verses are spoken by Eliphaz the Temanite who does say things that are incorrect and so these verses may be unreliable. It may be wiser to say that Eliphaz does state a truth here and that is that although there is a holiness about the angels that is far above the greatest examples of holiness among men, nevertheless it is less than the holiness of God. He can discern an imperfection even in the holy angels.

Angels shouted for joy when God created the world

Job 38:6, 7 On what were its footings set, or who laid its cornerstone - while the morning stars sang together and all the angels shouted for joy?

God questions Job as to where he was when God created the world. This leads to the reference to the angels adn creation. We know that the angels and all heavenly creatures were created before the creation of this world. Here we learn that the angels shouted for joy when God created the world. The morning stars must be another way to refer to the angels. Jesus is the bright and morning star (Revealtion 22:16) and they are morning stars. Stars speak of glory, purity and light and of constancy, permanency and the numbers points to the fact there are so many angels.

It is rare to find a reference to angels singing in Scripture but that is the implication here. Perhaps the cause of their rejoicing was the anticipation of the work of salvation for which this earth forms the theatre. It may have been simply to do with the creation itself.