I am not sure if the correct term should be “phase inversion,” but what I need is the right channel to be exactly the opposite to the left, this being just like the original mono.
I know this may sound silly as usually they would null each other, but case is old home computer had mono inputs that will take the right channel as ground if a stereo plug is used. This inverted channel should give a raised amplitude.
I can do it by hand or in a step by step with intermediate files but I think is not efficient.