An oscilloscope is a cathode ray tube that has an electron beam that can be moved around by applying varying voltages to its inputs. In this case it works as an XY plotter, i.e., you specify the X co-ordinate by the value of the voltage on one input and the Y co-ordinate by the value of the voltage on the other input.

You can display lines on the oscilloscope screen by giving a specially generated two channel audio as its input; one channel works as the X co-ordinate and the other one as the Y co-ordinate.

The game (Darkplaces which is a source port of Quake) renders a scene that has the lines to be drawn (triangles, in fact) on screen. The output of the game, i.e., the lines to be drawn on screen are sent to an audio synthesizer process(PortAudio). This process uses a sound card to create the audio. The output of the sound card is connected to the inputs of the oscilloscope which draws the lines on its screen.

So it's not the sound in the game that is producing the actual image. The output of the game is converted to an audio signal which when given as input to the oscilloscope creates the actual image.