Hope you have as much fun and success with them as I have.
I managed to hide all mine behind the layout fascia, together with a DPDT switch,
to enable me to "move" the sound to different areas of the layout,
which have their own speakers.
I run my units off variable voltage regulated DC wall warts using the 9V setting,
and if your MP3 units are the same as mine, you should see the reverse side as this.
With power in, going to the red socket on the right,
and a three wire ribbon cable, from the right hand socket, of the pair on the left,
going to the amp.
Both cables should be included with the unit,
but you'll need to provide wire from the amp to your speaker.