Hey. can't believe noones replied to this yet...

anywho, this is a problem with your account permissions. If you go to the last blade on the dashboard (I'm taking this from the top of my head so... it's not gonna be perfect advice... now) and click on settings.. then look for some game settings, or account settings and find an option for "Play original Xbox games".. make sure that says yes. Make sure to check that you can play DVD's of all ratings and turn on anything else that's in there... hopefully that'll help... 

This isn't a song from in game. I'm not 100% sure either but this is the PS2 version of Guitar Hero.

What this guy done, is got a song which played like the Halo 3 tune, recorded him playing along to it and then mixed it with the proper song on his PC. Maybe, in the future this song will be available on the marketplace, but for the time being, it's not gonna be in game. Brilliant video though :D

- ACTUALLY, On watching it again, it seems too perfect.. I'm 100% sure that the song isn't in the game.. but, maybe he has a modded PS2 and created the custom song himself, one things for certain, the song playing along with the video was added in after it was recorded...

Don't be getting your hopes up :P