The intro video music
By searching on youtube, you can find all the musics used in every possible AC/ACII/ACB trailer, be it official songs or instrumentals. But I have never seen the one used in the introduction video to AC anywhere.
We can hear it particularly well in this video, because for some reason the Templar's voice has been muted in the PC version :
I know it hasn't been released on any soundtracks, but so is the one from the ACII E3 trailer, and it was on youtube without any of the trailer sounds several months after the release of the game.
Anyone has an idea on where/how to get our hands on it ?
well anywhere other than youtbe lol 
. but try on a website for game sound tracks or type it in on google
I did a lot of research on Google, in vain. That's why I'm asking here.
It probably doesn't have it's own track because there's not really much to it. It sounds like a bunch of sound effects with a medley of AC music playing at random times.
It never occured to me that it wasn't even a track. Though I'm sure the choirs beginning at 0:52 accompagnied by drums during the chase aren't part of any AC music. They must have recorded it for the trailer then left it somewhere to be forgotten. A pity. Thanks anyway for your answers.