Skin effect has zero role in signal transmission at audio frequencies. This is simply more snake oil promoted by cable manufacturers and their "believers." Cables are passive devices and as a result have a linear response. Only at very high frequencies, way beyond those of human hearing, does skin effect play a significant role.
Stranded vs Solid Copper
I am making a home made outlet box with Hospital grade receptacles. I am hard wiring into the existing outlet box that was poorly installed.(to far recessed) Does it matter if I use solid or stranded wire for the extension wire/cord in terms of its effect on sound? The length is 4’ to 5’