Since 1.11 WoW will no longer look in the data folder for model/sound swapping.
The workaround?
1. Open mywarcraftstudio
2. In menu go to: pack -> create mpq archive. Call it whatever you want, doesn't matter
(EDIT: Apparently it must be called the same as an existing mpq, but with a number 0-9 behind it eg. speech2.mpq or patch2.mpq.)
3. Add your files to swap into the mpq, make sure you enter the right folder location
4. Open WoW
Tested with my swapped fire elemental voidwalker, and it works fine