Why haven't they gotten sounds yet? They should be easy for the devs to make! Hell, I made my own: http://www.mediafire.com/?6oi1fw34t32p45r but I don't know how to put them into the game D: It's been so long since villagers were added, yet they don't even at least give them player hurt noises...
My assumption is never. This is how Minecraft works. They work on something for a little while. Then, when it is nearly done... they completely drop it. Never to be improved upon again.
I agree that they do need sounds. But because of how long they have been in the game, them having sounds, would just sound weird. But it would be a good weird. I don't think that these are a priority to Mojang at the moment. They have a lot going on like, realms, snapshots, xbox edition, mcpe, etc. I do though, think that horses should have noises before villagers and witches. It's just something that would seem better.
I would love it if you checked out my "about me" page on my profile and looked at my threads I'm always happy to see new posters.