Here's why:
* Sound: "Son-" comes from the Latin word "sonus," meaning "sound."
* Write: "Write" comes from the Old English word "writan," which itself comes from the Proto-Germanic word "writaną," meaning "to scratch, to engrave, to write." The root "son-" relates to the idea of "making a mark" or "creating a visible impression," which is similar to the act of writing.
Therefore, the root "son-" is connected to both the concept of sound and the concept of creating a visual mark.