Some info I've overlooked until now - solo vocals for ACR main theme and other Balfe cutscene tracks is performed by Madeline Bell who won the contest. More details on that:
http://www.ujam.com/campaigns/acreve...s/introduction
As you can also see in those screenshots of music credits, she is mentioned alright, but I really skipped that part...
And one more thing - this wasn't specifically mentioned in the credits, but anyone who reads the forum posts can find it useful I guess: solo female vocals performer in Kyd's in-game tracks is always Melissa Kaplan (kind of a tradition since AC2), and as already mentioned, that's Madeline Bell for Balfe's tracks. However, there is no performer credits for solo male vocals performer in "Die By the Blade" track, so that remains...a mystery lol