Does the pronunciation of 学生 resembling Gaksei rather than GaKUsei reflect voluntary shortening of words? Why do certain phrases like 奨学金 audibly pronounce the KU sound?
It's called devoicing and it generally affects the vowels i, u between certain consonant sounds. You may be aware of this from the pronunciation of です and ます. It's not fully predictable, so you have to just memorize how the words are pronounced.
It also occurs in fast and slow speech alike, so it's not shortening but rather just how the words' pronunciations came to be in the language.
And to clarify, this is a different thing from gemination, which occurs with certain sounds merging with the next into a geminate consonant, like 学 がく + 校 こう = 学校 がっこう. This is also not all that predictable either.
