What is the subtitle of the free Portal 2 soundtrack released by Valve?

The subtitle of Valve’s free Portal 2 soundtrack is Songs to Test By.

Valve released Songs to Test By as a free digital soundtrack in volumes between May and September 2011. It contains nearly all of the game’s instrumental music, including pieces built around Portal 2’s dynamic system, which changes or develops in response to actions inside the test chambers.

The title is a playful extension of Aperture Science’s testing language. It also distinguishes the release from the game’s individual songs, such as Jonathan Coulton’s “Want You Gone” and The National’s “Exile/Vilify.”

A retail four-disc Collector’s Edition followed in 2012. That edition reused the Portal 2 music and added music from the original Portal, but the free Valve release is still known by the subtitle Songs to Test By.