Nix is the art of containers without actually using containers.
You can mix unstable and stable packages quite easily on your system without fear of getting your system into a unrecoverable mess.
In this case you have aaxtomp3 from unstable, and leave the rest of your system on stable.
@TLATER has a in depth explanation of this. Installing multiple packages from unstable channel in configuration.nix - #2 by TLATER
I’d avoid using nix-env to do it, and just do it from your system configuration.
If your using flakes for your system configuration , it’s even easier to add an inputs to stable and unstable nixpkgs, and pull in packages from both.