As another option (while not wanting to take a job away from Phil!), the simplest possible option - although definitely NOT the cheapest - is to pick up one of the LGB "black box" all-in-one sound modules, the 65000 series (the one you'd want is 65001, the American Steam module). At list price these are quite expensive (£150-ish), though you can sometimes find second-hand ones on evilBay - it's often possible to find one for around £100 or so if you have a bit of a search.
While there are certainly better and/or cheaper sound systems available, the advantage of the 65001 for the raw beginner is its simplicity - the electronics, speaker and volume control are all housed in the one little black box (about 60mm square and 20mm deep), and the only thing you have to do is to find somehwhere to physically put the box (inside a big tender shouldn't be a problem at all) and then connect two wires from it direct to track power. It will then work on analogue or DCC, automatically sensing which it is picking up.
You don't have to worry about mounting a separate speaker somewhere, or insulating/protecting a bare board of electronics, or anything more than the two-wire connection.
Just offered as an idea, should you decide you want something ultra-simple that you can install yourself......
Oh, and welcome to the forum!
Jon.