There are a lot portable mini speaker out there, but this Mighty Mini Speaker that in mini boom box and guitar amp shape, are the smallest and lightest that I ever see.  As you can see, you can even attach to your keychain.

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There is a built in USB-rechargeable battery pack at the speaker. Therefor instead of consuming the power from your mp3 player batteries, it got it own resource to boost it sounds. On the other words, the speaker should have plenty of sound out of them. Charging is pretty easy, just  hook it up to your computer and it’s done. A full charged of batteries will run for up to  2.5 hours and the 3.5mm audio jack is where you connected to iPod or other portable multimedia player.