Here's why:
* Diphthong: A diphthong is a sound formed by the combination of two vowel sounds within the same syllable. Examples include "oi" in "coin," "ou" in "house," and "au" in "haul."
* Sound: The word "sound" has only one vowel sound, represented by the letter "ou." While the spelling might suggest two vowel sounds, it is pronounced as a single vowel sound.
Therefore, "sound" is not a diphthong.