nixos: disable sound by default, if stateVersion >= 18.03 (#35355)

nixos/doc/manual/release-notes/rl-1803.xml

···
     </para>
     <itemizedlist>
       <listitem>
+         <literal>sound.enable</literal> now defaults to false.
+      </listitem>
+      <listitem>
         <para>
           <literal>matrix-synapse</literal> uses postgresql by default instead of sqlite.
           Migration instructions can be found <link xlink:href="https://github.com/matrix-org/synapse/blob/master/docs/postgres.rst#porting-from-sqlite"> here </link>.

nixos/modules/installer/tools/nixos-generate-config.pl

···
   # Enable CUPS to print documents.
   # services.printing.enable = true;
 
+  # Enable sound.
+  # sound.enable = true;
+  # hardware.pulseaudio.enable = true;
+
   # Enable the X11 windowing system.
   # services.xserver.enable = true;
   # services.xserver.layout = "us";

nixos/modules/services/audio/alsa.nix

···
 
       enable = mkOption {
         type = types.bool;
-        default = true;
+        defaultText = "!versionAtLeast system.stateVersion \"18.03\"";
         description = ''
           Whether to enable ALSA sound.
         '';
···
 
   ###### implementation
 
-  config = mkIf config.sound.enable {
+  config = mkMerge [
+    ({
+      sound.enable = mkDefault (!versionAtLeast config.system.stateVersion "18.03");
+    })
+    (mkIf config.sound.enable {
 
     environment.systemPackages = [ alsaUtils ];
 
···
       ];
     };
 
-  };
+  })];
 
 }