Oftentimes, what determines the pronunciation of one word is the word that immediately follows it. In the examples you cite (y usted), the Y gets buried because it's cumbersome (and unnecessary) to make two separate sounds out of those words, so the Y tends to slide right into the next word, resulting in something that sounds like "yoosted" or just plain "usted." You'll notice that this is usually the case when Y is followed by a word that begins with a vowel. In the second example, Sandra begins with a consonant, so there is no sliding from Y to the S. It requires two separate sounds. We do similar things in English. For example, the phrase The End is actually pronounced Thee End because End begins with a vowel. If you were to say The End (pronounced Thuh End), it would feel clunky. But if the following word begins with a consonant, the Thuh form of the pronunciation would apply. Example: The Boy. We wouldn't say Thee Boy. All of this goes to say that every language does this. Certain sounds are modified to make them easier to pronounce.