OO Vowel Sounds Vowel sounds are often difficult for non-native speakers to pronounce. Vowel sounds are made by slight changes in the position of the tongue and lips and tensing or relaxing the muscles of the mouth. You may be working on tongue position and movement in order to improve your accent. By prolonging or lengthening some vowel sounds, many words will sound more American. Perhaps the single most important factor in modifying vowel sounds is the ability to hear the sounds correctly.
Vowels are generally more difficult to master than consonants.
In addition to the problems created by spelling that are explained in the first paragraph, vowel sounds are made by slightly changing the size, shape, and tension of the muscles of your mouth and lips. A very small change can result in an entirely different sound. The OO sound is relaxed. That means the muscles of your mouth are not tense. It tends to be shorter than the U, too.