HIVE

I'm not sure why, but if I had to venture a guess, I'd say it is because they didn't want to rely on something external to allow sounds to be heard. MIDI is basically like sheet music for the computer--it doesn't actually record sound. It only keeps track of the notes, and the operating system will play those notes with "digital instruments" that they have. e.g:
http://imgur.com/a/mYYk7

And different operating systems will use different sounds (so it will sound different across OS's). Some users might not have those sounds installed at all! So I guess it was just a weird logistics issue, and they decided to scrap it altogether.

Nice explanation! Thanks for taking your time. I really hope they restore that option soon - I don't know if I'm the first person you see saying this, but I really enjoy those background sound effects, while many try to disable them lol.

Are you referring to an issue caused by the latest patch? I haven't tested the latest patch that much, but if you know which background sound effects no longer work, feel free to let me know. I'll take a look at it.

oh, I meant the ambient sound like northrend day/night, or when you could hear those birds and dogs doing their noises in maps with dalaran/cityscape tileset.

Fyi, Warcraft uses the Miles Sound System v6.1d. It is the file mss32.dll. The pre-recorded ambient sounds are in the DLS file format. They are all in the MPQs

In case you want to hear the ambient sounds without the need to run the game:
https://www.hiveworkshop.com/pastebin/da6a360954a7e821ce39ca939ff9e54c6426/
(v7.2g should also work on 64-bit OSes?)

Simply load the right pair of midi/DLS files.

Tbh, I really enjoy those ambient sounds, and you providing a way to listen to them again is awesome. Thanks for the help, pyf!