Its called a Line Out Converter. Couple resistors in a simple circuit, typically about a 10k Ohm resistor that drops speaker voltage down to line voltage, and another roughly 1k resistor that provides some load. You could build one yourself like I did for about two bucks. Literally, I think mine was a little less than that. Or you could buy one from Parts Express or a zillion other places for around $10-20. Search for LOC you will see.
Here is the clearest most complete description I could find. This is what I built mine on. http://www.epanorama.net/circuits/speaker_to_line.html
Here is the clearest most complete description I could find. This is what I built mine on. http://www.epanorama.net/circuits/speaker_to_line.html