4,000 Degrees Kelvin - the song from Portal which plays at one of the most tense moments in the game. It fits the scene perfectly, especially with GlaDOS' comments.
You gotta lotta guts mentioning Portal without putting Still Alive up here, too...
The first time This Troper played through Portal, she didn't even consciously realize there was music. It wasn't until she specifically looked up Orange Box music that she realized there was music the whole time--it's just so subtly woven into the ambient noise, it's barely even noticeable, except at a few very key moments (the radios and Still Alive, of course, but also the end of Chamber 19, getting the fully-functional portal gun, and so on).