So, I believe I found a fix for you. I found this thread, and tested it. I added "SET Sound_EnableDSPEffects “0” " to the line, restarted my game and my death knight voice filter was gone, I sounded like a regular human. I then changed the “0” to “1”, reloaded the game and the voice filter is back.
So if you do the same, go into the config in your “WTF” folder and add SET Sound_EnableDSPEffects “1” , you should now have your death knight voice distortion activated.