As in "dogs" and the suffix "-es". I have learned later that the ending should be voiced, while everywhere I hear native speakers pronounce them voiceless. However, I followed the book and pronounce them voiced, but then people seems not understand me.
Unlike languages like Standard High German, English does not systematically devoice voiced obstuents in word-final position. In phrase-final position or before unvoiced consonants, however, the voicing is considerably reduced.
Thus the voiced z-sound in
This place is going to the dogs.
is far less articulated than in
dogs of war
In other words, if you hold two fingers on your throat where you can feel the vocal chords vibrating, you should still sense the vibration in the first two examples, but it will be neither as strong nor as long as in the third.