Graphics isn't the only important part of the game. Sound is a second sense used by human to orient in the world. Background sounds, music, speech, sounds of effects - all these things can make your game more vivid and realistic. Even sounds in user interface make application more attractive and comfortable.
As an audio engine Xors3d uses Squall sound system. This library isn't inferior to famous FMOD, BASS and other sound libraries. It proved oneself in a great number of commercial and non-commercial games. The main advantage of Squall is that it's free for use in any type of product, so you don't need to purchase additional license.