Accent reduction, made easy, drills for an American accent, Wellborn J., 1997.
GENERAL PRINCIPLES ON THE PRONUNCIATION OF ADDED FINAL "s." As you may know, most nouns in English show the plural by adding an "s" at the end of the word, and for verbs, the third person singular, present tense, adds an "s," as in he waves. If the sound that precedes the "s" is voiced, then the "s" is voiced and is pronounced / z /: labs, tags, waves. If the sound that precedes the "s" is unvoiced, then the "s" is usually unvoiced and is pronounced / s /: laps, tacks, waifs.
