In reply to @Minijackson:matrix.org
IIRC, this means that your plugins installed through users.users.mindtree.packages won't be found, since you don't have the path /etc/profiles/per-user/$USER/lib/lv2

which gives:

github:musnix/musnix/6eb5c1714fbb7622b7270be78243365f9c55c9cb
├───nixosModule: NixOS module
└───nixosModules
    └───musnix: NixOS module

{
  description = "...";

  inputs.nixpkgs.url = "...";
  inputs.musnix.url = "github:musnix/musnix";

  outputs = { self, nixpkgs, musnix }: {
    nixosConfigurations."myConfig" = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";

      modules = [
        musnix.nixosModules.musnix
        # ...
      ];
    };
  };
}

I had to do some minor changes (update to the latest version), but it seems to have worked. For reference, here's what I ended up with:

{ stdenv, libuuid, libxcb, expat, gtk3, glib }:

stdenv.mkDerivation {
  pname = "Zebralette-mini-Zebra";
  version = "2.9.3";
  src = builtins.fetchurl {
    url = "https://dl.u-he.com/releases/Zebra2_293_12092_Linux.tar.xz";
    sha256 = "sha256:1imn0a1pjgqihcms86m7wblc9fifmc2l9xczykl8c0bkzw883xpx";
  };
  dontBuild = true;
  sourceRoot = "Zebra2-12092";

  installPhase = ''
    mkdir -p $out/lib/vst
    cp -r Zebra2/* $out/lib
    ln -f $out/lib/Zebra2.64.so $out/lib/vst/Zebra2.64.so
  '';

  postFixup = ''
    patchelf \
      --set-rpath ${expat}/lib:${libxcb}/lib:${libuuid.out}/lib \
      $out/lib/vst/Zebra2.64.so
    patchelf \
      --set-interpreter $(cat $NIX_CC/nix-support/dynamic-linker) \
      --set-rpath ${gtk3}/lib:${glib.out}/lib \
      $out/lib/dialog.64
  '';
}

However, when trying to run the plugins (in Bitwig), I'm getting these messages: