This is a speaker module. I bought it as a DIY-kit from Velleman for about 15 euros.
It's a stereo amplifier, and I have two inputs on the front panel. However, if nothing is inserted in the second input, the first input goes to both channels. The speaker is connected to output 1, and output 2 goes to a jack on the front panel. Output 2 is connected via the switching jack to a 100 ohm 2W resistor to ground.
Unfortunately there are no building instructions for this module, but at least you'll find the schematic, some photos, and some comments.
The finished module.
That's it! Now you can hear your patches!