I run the cables all outside the house, under the garden.
They just penetrate the wall and connect to the outlets of my listening room.
So the cables are not in-wall
Flexible cords and flexible cables
400.12 Uses Not Permitted
(1) as a substitute for the fixed wiring of a structure.
(2) Where run through holes in walls, ceilings, or floors
I use to use cord in my system. It was Cullen OFC Cord. I ran it directly into a power strip. No receptacle at the wall. Just up and out of a hole and into the strip. Pretty illegal. This was years ago when I started trying to figure out how best to power a stereo system. I figured the power cords from the power strip to equipment were the disconnect if an issue occurred. It worked really well. Better than plugged into a wall outlet fed by 12 AWG Romex. This is when I discovered, every time you have a termination, you introduce dissimilar metals as well as noise and additionally block RF from being drained from the system. And vibration control is critical. Any old box in the wall will not do.
A friend then got a pile of Furutech #10 cord and he powered his system the same way. It also worked very well. We then installed #4 NM-B to a new subpanel at his house. From the subpanel, Oyaide direct wired from the panel and out to the gear with a Bochino on the end. That was pretty amazing. Again, not legal. But way better than most any $3000 power cord to a receptacle in the wall in his system.
Since his cord was out of the system, we took it to another project where I installed a new subpanel and new branches circuits to the room. We wanted to play around so we took the Furutech cord and direct wired it to the panel, put a nice Furutech end on it and plugged it into the customers amp. Something was not so good. A bit flat. We unplugged it and put an AQ Hurricane cord into an AQ outlet that was fed by one of the new branch circuit, then plugged that into the amp. Holly crap that kicked the crap out of the Furutech. It was one of the most dramatic difference I have ever heard.
At that point I was moving away from stranded wire to solid.
Its highly unlikely you will have fire. But the installation is not to code and highly irregular. And I really think you will get better performance removing the cord and going with twisted #10 THHN. Of course, that is only the tip of the power supply. Its the visible part of upgrading power performance. There is a lot of hidden work in panels and grounding as well as vibration dampening that influence electron flow and harmonic/RF disturbances as much or more than upsizing wire and using dedicated circuits.