Try mine instructablehttps://www.instructables.com/id/Portable-speakers-Speakers-on-battery/
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Look at my Instructablehttps://www.instructables.com/id/Super-easy-to-make-portable-speakers-for-mp3ipod/
You'd need a stereo audio amplifier. This is a good one:https://www.instructables.com/id/Headphone_Amp_Chu_Moy/L
Exactly like I was going to say. However, it should be noted that this will only save you a little bit of battery power on your mp3 player(which i assume you're trying to do). The way to do this is turn the volume down low on your mp3 player and turn up the volume on your amp.
I think that the speaker is a heavy-load on the mp3 player and it keeps running out of charge, or the volume isn't loud enough. Unless the amp is taking a lot of current (which it shouldn't) the volume level isn't going to make a lot of difference to mp3 battery life, but I agree with your advice.L