Can only guess, but nixpkgs says the package provided is org.gnome.decibels not decibels. Maybe try this command

It is called pkgs.decibels. Should be feasible if you just did

  environment.gnome.excludePackages = [
    pkgs.decibels
  ];

Ubuntu -> Mint -> Arch -> NixOS

Ubuntu really drove me away for some time. Only when I got to Mint, I felt that Linux could be a daily driver. Arch made me strangely reinstall it every few days. Not by not working but by having the feeling of dirtying the machine on use.

NixOS is really what I want to use the rest of my life. Very complicated on first use but definitly worth it (for me).