Here's why:
* Common nouns refer to general things, not specific individuals or places.
* Sound, as a concept, is not specific. It refers to the general phenomenon of vibrations that can be heard.
Examples:
* "The sound of the waves crashing on the shore was soothing." (General concept of sound)
* "The sound of her voice was sweet." (General concept of sound)
However, "sound" can also be a proper noun in some cases. For example:
* "The Sound of Music" (referring to a specific musical movie)
Therefore, the classification of "sound" as a common or proper noun depends on the context of its use.