Valve has released a good part of Portal 2’s soundtrack for free. It’s at the link above. There are also ringtones. I think they’re more useful as a preview for the uninitiated, but then again, I’m a default ringtone kind of guy.
To me, one of the most memorable aspects of Portal 2 was how its music always seemed in lockstep with the game’s pacing and art direction, and especially with its detailed, otherworldly sound design, but taken on its own this is still a wonderfully textured, playful, varied score, for the most part matching the urgency of an action film with skittering beats and glitchy, buzzing synth. Some sections are disarmingly sparse and downtempo, while others veer toward the ambient, sounding more like they were produced within the world of the game by a lonely AI. Robots making music is arguably a part of Portal 2 already, so it’s fitting that the conceit is embraced by the MP3s at the link, which are attributed to the Aperture Science Psychoacoustics Laboratory. Listening to this score, it’s easy to imagine the psychoacoustic lab’s sentient mainframe monitoring your performance as it samples Aperture’s industrial sounds, breaking its silence only to issue iterative loops calculated to slow you down, drive you forward, or impede puzzle-solving. This isn’t part of the game, but it’s to the credit of this soundtrack and Valve’s world-building that it could be.