Hi,
USB is digital and 3.5mm outputs are normally analogue audio... So a simple cable won't do it?
I guess your MP3 player has a 3.5mm output, then your speakers have a male 3.5mm plug? Then you just need a 3.5mm coupler to convert it back to a female socket, with one of these:
http://www.cables-lea...pler-female-to-female
Then just grab the length of audio cable you need, like these:
http://www.cables-lea...uk/3-5mm-stereo-cable
Best place I found for cables!
Hope this helps ;-)