What is true regarding the voicing of vowels?

The assertion that all vowels are voiced sounds is accurate because, in phonetics, vowels are produced with the vocal cords vibrating, resulting in a continuous sound that is rich in harmonic qualities. This vocal cord vibration is essential to the definition of vowel sounds, as opposed to consonants, which can be either voiced or voiceless.

In languages like English and Spanish, every vowel sound is characterized by this voicing; there are no instances of voiceless vowels in standard phonetic practice. Voiced sounds entail the vocal folds coming together and vibrating as air passes through them, a defining characteristic for all vowel production across various languages.

The other options propose ideas that don't hold under scrutiny; vowels are not voiceless nor can they vary in voicing the way consonants do, as the nature of vowel sound production is fundamentally dependent on vocal fold activity.