Morphologically considered is the initial sound of the first sound of a word.
So "n" is the initial sound of the word "name".
The initial sound can be followed by a medially or directly from a final position, as in "you".
Generally there are more sounds as letters of a word. The initial sound must not be confused with the first syllable of the word. The initial sound of a syllable is explained by syllable head.
Initial sounds are combined with appropriately matching images, often used in elementary school in the form of so-called Anlauttabellen in the first classes to language acquisition.