There are only 26 letters in the English language but there are 44 different sounds. One example of a sound without a letter is the ‘ng’ sound as in the word ‘singer’. This sound is designated by a special symbol that is called engma. French, German and many other languages have additional sounds in languages that English does not use and each sound has a special symbol (unless it is associated with a specific English letter) used by linguists.

(The Great Courses – Understanding Linguistics: The Science of Language)