You will need to power the speakers, somehow.
http://oscarcontrols.com/speaker/index.shtmlThat link will take you to a hack that is cake. I ordered a 4-inch speaker set from The Real Bob Roberts that are 8 ohm shielded speakers. Make sure you get shielded, or it could mess up you monitor. I think you can buy them individually, too.
The hack is super simple, and works well. There are other ways to sneak the sound control into your cab. I have mine hidden under the player one CP on my cocktail. It won't matter if you stay with one speaker, or not. The only difference is you won't get stereo sound for games that have it. If you're going with only old-school games, it won't matter.
Hope that helps.