No, the word "sound" is not a diphthong. It is a monophthong, as it contains a single vowel sound, represented by the 'ou' diphthong in this case.

It's called a diphthong literally, checked out a dictionary it will explain it a little more

A diphthong is a word that has two vowels in a single syllable. The sound moves forward from one vowel to another in the word. An example of a word that has a diphthong in it is the word feat.

a diphthong

No. A diphthong is a combination of two vowels that make a sound which glides between the two, but sounds like neither. The word Out contains an OU diphthong. The letter Y often combines with other vowels to make diphthongs, such as Boy, Toy, Joy, etc. However, by itself it cannot be a diphthong.