Neutral vowel sounds, also known as schwas, are the most common sound in spoken English. They are unstressed, short, and often reduced vowels that occur in the middle of words. Schwas are typically represented by the letter 'e' in spelling, but can also be represented by other letters such as 'a, ' 'i, ' and 'o.' They are used to create a smoother flow of speech and to make pronunciation easier.