Tonight, after logging off the game, I accidentally clicked on the credit panel, and found that there was a change here. In addition to changing the background image, instead of the usual in-game sound (I think it was a Evendim theme), now we can hear the real new song. I would say that Tolkien wrote verses and Chance Thomas made music, only the performer is unknown to me ...? who is that singer? And how long is this song in the game?
I have noticed that the credits themselves, have not changed, and are the same as that of Turbine staff years before...
The singer is David Osmond. The song was released with the Mordor soundtrack - http://www.hugesoundrecords.com/mordor/ - and added to the credits around the time of the Mordor expansion release. So last July/August? Something like that.