UberDark is right even if it is often big publisher (EA for example) having agreements with music publisher (universal for example) and they both know that "big" music beeing used in "big games", and having "big games" using "big music" reinforce both.
This is all marketing both of the time.
And sometimes there are indys like "Insanely twisted shadow planet" who will have agreement with lesser music publisher ("Nuclear Blast") and have Dimmu borgir as sound track.
From the moment the music is published, you have to deal with the publisher.
Finaly "streaming from youtube"... well, youtube streams video, not music only so it's like downloading more than required and I'm not sure it is legal as you require the authorization of the video author (as long as his the right-holder for the video and the soundtrack).