GOG offers better Voice Acting, better Sound and better HD Textures for free. This is fine, but why weren't these part of the original game or part of an update. Why the splitting in pieces?

I suppose either because they required optimization post launch to be included, or because some people prefer the smaller download and install if they can't run it anyway.

Actually, I have to point out that the developer did not offer "better sound", in the way that most people assume. Unlike visuals, for which we have not reached the point of perfection and thus there is no such thing as "full quality", audio is either "full quality" or something less than that. I'm not talking about uncompressed audio, I'm talking about audio for which 99.9% of people can not perceive anything better.

In other words, there is no technical reason for audio to be of perceptibly inferior quality. If the dev is offering "better sound", it simply means that the original audio was of inferior quality to begin with.