I´m reviewing telling time. Listening to pronunciation of Duas horas and Três horas, I noticed the letter “S” normally sounds like sh at the end of a word. When hearing Duas horas and Três horas, the “S” in duas and três sound like a “z”. Am I hearing it correctly? Is this because H has no sound and the next letter is a vowel? Therefore you have “S” between two vowels forming the “z” sound.
@jhramsey68, yes, that’s right 
Any S between two vowel sounds, even if from different words, may sound like a Z when pronounced at regular speed (also depending on the regional accent). That’s what happens in “Duas horas” and “Três horas”, since the H is mute.