The short answer is there is a phonological rule in Japanese, that when a high vowel, /i/ or /u/, is sandwiched between two unvoiced consonants such as /s/, /k/, /t/, /ts/, /sh/, etc, it becomes devoiced. Yu
suke and Sa
suke both meet this condition, so the u is not voiced.
You can search through the board for "devoicing" or "
muted" for more threads that talk about these things