Thank you for all these answers and ideas for solutions! It helps me a lot to see more clearly what I can do with Unreal’s current tools, and what problems I might encounter.
My priority is the feeling of live interaction, so to have the shortest possible delay (no delay, or at least no sensation of any delay would be the best).
So I have to turn either to hardware, or to Max/MSP or Pure Data or SuperCollider, or maybe to C++…
In the meantime, to minimize the risk, I will start perfecting tests with the Audio Analyser plugin I recently found on the Marketplace, and pre-recorded human voice samples. ^^
Thanks again! I’ll keep you posted if I have any questions, additional requests 
Btw,I hope that all the beautiful things you show here will one day be in the engine!
Long live UE! 