There is no advantage adding sound to a loco that already has DCC. You must remove the existing decoder to install the sound decoder. In N, there are not any "add on" sound decoders like are available in HO.
Adding a non-sound decoder to the older 2-8-0 is easy. There is a removable weight in boiler under which all the wires are connected. It makes things pretty straight forward except for the fact that the wire color coding is backward on a few things.
As far as adding sound to the 2-8-0, it is probably easier in the older version because there is much more room inside the tender. In the newer versions, the metal floor and lightboard or DCC board, depending on the version, take up a lot more space and have the pickup leaf springs built into them. That means you must use the lightboard or completely rebuild the pickup system. It really doesn't matter too much though, any of them can have sound installed.