I have been looking all over the web these days. Not just for sheet music for POTC, but for something else.
Search enginges barely do good these days. You must pay money (usually) to have your site appear when a tag is typed in. So I looked the old fashioned way; looking.
Few websites that deem themselves "Halo encyclopedias" are just that. It took about an hour. not to easy. But I found it.
The script of Halo 3 (or,not script) is long. Very long. I took me two hours to read it, and doesn't battle. It's just the cutscene scripts. And they're very lengthy, especially the text said by 343 guilty Spark, and 01 Shamfully Anomly. At one point, I was actually glad the Aribiter, whom I hate, is playable because an Arbiter level cuts off part of one of 343's long chats.
The website I found the script on had one page, with two items on it. 1 is a small note, saying what you would be looking at for the next hour. And below it was the script.
That website, whose link I never got, I cannot find. I found a mirror of it though.

http://www.halo3xbox.com/leaked-halo3-storyline.php

If it's a fake, then this person had WAY too much time and has a VERY good understanding of the series. Though, I would not be surprised if it was loosely wrong in some areas.

There is one little snag:
There's a cliffhanger(sort of)
The Ark, of, Installation 01, is destroyed right outside earth, and the Arbiter and some of Install 01's peices are sent into the depths of the Indian Ocean. At the bottom, the Aribter's lifeless body floats with the peices of the control room. But, Cortana, who has experienced rampancy and is hazardous to the Covenant/UNSC Alliance and the Forerunner.
At the ocean, a peice of the control room (that has Cortana) sinks down next to the Arbiter. then, a blue whiusp, laughing, enters down the Arbiter's throat, and the Arbiter then breathes. Then "Credits Roll"

You finish the fight. Right.