I take it that you JUST want to add sound that will work on DC, Bob, not upgrade to DCC at the same time?
There are always the LGB "black box" (65000 series) self-contained sound modules, but although these are very simple to use in terms of wiring they do have the disadvantage of being quite physically large - in a loco like the Forney there proabably wouldn't be any option except to put one in the cab, where it is a bit visually obvious and even then it might be a tight squeeze.
However a much smaller (and cheaper) option that is almost as easy to install would be a Massoth "S" sound module, which you could buy ready programmed with the right sound files for your Forney; the electronics module itself could be hidden away inside the bunker space with a suitable speaker, and the only wiring needed is two wires direct to track power and the the plug-in wiring to the speaker. The module will work on either DC or DCC, though obviously on DC you don't get direct control over things like the whistle and bell - though these can be triggered with reed switches and track magnets if you want to fit them as extras.
I'm sure several others will add alternative suggestions, there are a number of ways of doing what you want at varying levels of cost and complexity - I'm only offering what would be my preferred choice in your case.
Jon.