Play the MP3 into the mic jack...........
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Right, or if you are half way proficient with tehcnology and devices, you can do this more directly in one of two ways. You can play the audio into the microphone input of your sound card externally, but the only trick to that is to make sure that the played audio is not over modulated (tweak the volume level), or actually route the audio signally internally in your audio card software and driver settings.
Yes, keep the level low, lower than what you would use to listen...............