Cell phone speakers, commonly known as “sugar cubes”, have become quite popular in smaller scale sound installs. They put out a surprising amount of bass and volume considering their tiny size. These also might be useful in small O-scale locomotives and critters.
Larry Hanlon recently posted to the Loksound Yahoo group some very helpful information on how to get the best out of these speakers. You will have to go there to get his .PDF discussion, but here’s a direct link to an excellent video he produced. Just be sure to listen through headphones or very good external computer speakers. Many differences will be hidden if you listen through built-in tablet, laptop or computer speakers.
Note that the sample response curve shown at 1:25 is actually a Tang Band 1931S module.