[Galin]

Originally Posted by Flame of Anor

This may have already been covered, but i was thinking, since Yavanna sung about ents and they were made by Eru, and Manwe sung about eagles and were produced, maybe Morgoth sung about Orcs in the Music of the Ainur, and though it may sound bad Eru created them because as he proclaimed during the music (...)

Tolkien did once contemplate:

'Out of the discords of the Music -- sc. not directly out of either of the Themes, Eru's or Melkor's, but of their dissonance with regard to one another -- evil things appeared in Arda, which did not descend from any direct plan or vision of Melkor: they were not 'his children'; and therefore, since all evil hates, hated him too. The progeniture of things was corrupted. Hence Orcs? Part of the Elf-Man idea gone wrong. Though as for Orcs, the Eldar believed Morgoth had actually 'bred' them by capturing Men (and Elves) early and increasing to the utmost any corrupt tendencies they possessed.'

Myths Transformed Text VII

This was not the only idea Tolkien mused upon concerning Orcs. Whatever notion he was going to land on (and possibly he was going to present conflicting ideas depending on where the ideas came from, or maybe he was going to leave the matter vague), here are a couple examples of what Tolkien himself had already published -- so in my opinion he would have to mind these statements in any case:

'The Orcs were first bred by the Dark Power of the North in the Elder Days.'

Appendix F

'No, they eat and drink Sam. The Shadow that bred them can only mock, it cannot make: not real new things of its own. I don't think it gave life to the orcs, it only ruined them and twisted them;...'

Frodo, The Tower of Cirith Ungol

Treebeard implies that Orcs came with the Great Darkness (first quote), and earlier he had given a general idea (at least) of when this Great Darkness began:

'It is a mark of evil things that came in the Great Darknesss that they cannot abide the Sun; but Saruman's Orcs can endure it, even if they hate it.'

Treebeard

'They always wished to talk to everything, the old Elves did. But then the Great Darkness came, and they passed away over the Sea, or fled into far valleys, and hid themselves,...'

Treebeard

I'm aware of Tolkien's statement in a letter that Treebeard is a character in the story and cannot be expected to know the exact origin of Orcs, however he is stated to have a great memory too, and I think the reader can regard these statements as reliable enough. Treebeard does not, in any case, state what Orcs were made from, but rather that they were made in mockery of Elves, which does not necessarily mean they were made from Elves. This left some room for JRRT to jot down various theories about Orcs with respect to what they might be corruptions of, including beasts, Maiar, Elves, Men, all of which can be found in texts also under the Myths Transformed section in Morgoth's Ring.

A mannish theory can be found in one of Tolkien's notes to the essay on the Druedain published in Unfinished Tales, where (it is stated that) the Eldar said that surely Morgoth bred Orcs from various kinds of Men, but yet not the Druedain.