Wishbone: They don't, and they have. HOMM2 for example, used to come with the original redbook audio. Now it comes with .ogg rips. I have no idea why, and I certainly haven't seen a post from GOG about it.
Fuzzyfireball: There is a sticky in the HOMM forums, saying they have updated the .exe
Perhaps that is what you were talking about?
I'm sure it is. However, I'm not about to check each and every game forum looking for a post that may or may not be there, about updates to the installer.
Besides, that post still doesn't say anything about why the redbook audio was removed. In fact it doesn't mention it at all.
They really should make update information available on the game page itself. A changelog would be nice too. And they should make a post in the general forum when they make updates. They make one whenever they add a new wallpaper to a game, why shouldn't they do the same when they update the game itself?