SOLVED:
So it seems I was just doing the crazy over-complicated method due to my lack of knowledge in audio.
It was suggested on a facebook UE4 developers group to use EQ to create the same effect.
So I toyed around, not really fully understanding the controls, and eventually changed the EQ Gains to something low like 0.16…that’s it.
I also stumbled across the Set Sound Mix Class Override, which I could use to transition between a normal sound mix and the deafness one, which is great. The things we learn 