Final answer:
The sound property most akin to brightness of light is loudness, which relates to sound's intensity level, just as brightness does to light intensity. Timbre, however, refers to the unique quality of sound from different sources.
Step-by-step explanation:
The property of sound that would be most similar to the brightness of light is loudness. Loudness is related to the intensity level or decibel of a sound, and it is similar to how brightness in light is related to the intensity of the light. Light brightness is to visual perception as sound loudness is to auditory perception. However, the question seems to be addressing the quality of sound that makes different instruments sound unique when playing the same note, which would be timbre. Timbre refers to the qualities or properties of a sound that enable us to distinguish different sources of sound when they are playing the same note at the same loudness. Terms like dull, brilliant, warm, cold, pure, and rich describe timbre, indicating its subjective nature and reliance on perceptual psychology.