There are several options, and everyone will have their own personal preference - but for both good sound and simplicity, mine would be a Massoth S, simply connected to track power with two wires. A small enough module (about 20x35mm) to install almost anywhere, and can drive a speaker of anything from about 25mm diameter upwards (use the largest that you can get to fit).
On analogue DC you will get all the "running" sounds while the loco is in motion, plus a toot on the whistle when it pulls away; if you are careful on your controller knob, you can also find the "sweet spot" just after the sound cuts in but before the loco starts to move, when the loco will sit stationary but still making all the "standing sounds" - steam hiss, various pumps etc. If you want to be able to trigger bell and whistle from track magnets you can also install reed switches under the loco which connect quite simply to the S board.
Jon.
Edit: sorry, just looked up which loco you mentioned (LGB 23430), didn't recognise the number without checking - my suggestion still applies, the S unit will be available programmed with the correct sound files for that loco plus you will have plenty of room for a nice BIG speaker!
You'd also have room for one of the LGB "black box" 65000 series sound units, of course, which are in many ways even simpler to install - but the sounds on these are more generic, AND they are much more expensive than the Massoth module.....