I'm not an expert, but perhaps the output is much to hot for your input jack. Guitar amps are known to handle much more abuse than other devices. Is the device a lot louder in your amp than a guitar or other normalized source of input?
well the links to the mp3's don't seem to work for me but i'm guessing what you need is this topic:http://www.circuitbenders.co.uk/forum/index.php/topic,397.0.htmlor more specifically this diagram: