Place it right after the accusation in your edit, then follow it with Bruh for the disbelief and Emotional Damage for the fallout. That order turns the denial into a scene with a setup, a reaction, and a payoff.
このサウンドについて
This is a very short spoken line, lasting just over a second, in which Filthy Frank says he didn't see anything. The flat, quick denial feels funny and evasive, like a joke excuse after something awkward has just happened. It works as a deadpan reaction to being caught watching, noticing, or knowing too much.