Which files have been updated?

I don't know exactly which files have been updated, but changelog (new handy feature on your library) of The Black Mirror says that:

"GOG version game change (12.05.2015)
The recurring "choppy sound" issue that was fixed and returned with the return of the game onto our shelves has now been fixed...again. This applies to the English version of the game."

As far as I know there hasn't been new updates since then.