From Halopedia, the Halo wiki
The audio playground is a sandbox used by Bungie Studios to experiment with sound effects during the development of Halo 2. It consists of a large enclosed space of varied terrain in which the player can test hitting objects and gunfire against the floor and walls, which represent particular surfaces. The surfaces' names are frequently written in the texture pattern. Audio playgrounds have been shown to come in variants for "solid steel", "dirt", "room", ice, water, and reportedly can feature virtually every substance in the game.