The intensity of a wave is proportional to the square of its amplitude:
[tex]I=k\cdot A^2[/tex]In the case of a sound wave, intensity can be perceived as loudness, while in the case of an electromagnetic wave of visible light, the intensity is perceived as brightness.
Since the amplitude and the intensity are related, then the amplitud of a sound wave is to loudness as the amplitude of a light wave is to brightness.
Therefore, the answer is:
[tex]\text{Brightness}[/tex]