you can kill them without anyone being able to hear them scream "YourName IS A TRAITOR", and no one knows they are dead. I find that vanilla TTT is pretty well balanced, and this just obviously breaks the balance.
I don't really think it would break the balanced that much.
Lots of players, play the game successfully without using voice.
It would just make people to use the suit zoom messages more, and possibly people would even start paying attention to them.
I have just bound a button for "[name] Is the Traitor." and spam that, and it usually works if i get more than one lines of it spammed.
Also If there would be a whisper voice, there also should be an option for local chat.
So there would not be even more advantage to people who use mics.