Oddly enough, his MP3 player doesn't work from USB power. (I assume you are aware that most USB devices can be powered through the 5 volts coming through the cable which connects to your computer.) If you plug in the USB, it assumes you want to transfer files to or from your MP3 player, and it won't operate in "Play" mode, only in "Transfer" mode.
So the trick is to create a fake battery which can reside in the battery compartment but actually carry power from some other source.
It's not that hard to create a fake battery; just cut a dowel to size, put brass screws (borrowed from a handy wall plug), on each end, and run wires from the screws to whatever external power supply you happen to have handy.
Of course, the power supply must be able to output 1.5 Volts.
Those aren't readily available, unfortunately. The best we could come up with was one of those variable wall-warts (i.e. transformers) with a 3 Volt setting. James was already using it to power another device, so all we needed to do, was to devise a simple circuit which could convert the 3 Volts to 1.5 Volts.
That's where I came in.
I found a circuit in one of my old Radio Shack books that uses a very common LM317 voltage regulator, and created a prototype circuit board with appropriate resistors to transform the input voltage to a steady 1.45 Volt output voltage. Easy as pie.
James found some PVC pipe and a rubber end cap to encapsulate the device. And the we attached it to the fake battery.
Then we put it all together ...
... and there it is! The adapter of his dreams!
And now James can listen to his music without using up batteries!
NOTE: Wall-wart and MP3 player battery case cover not shown. Yeah, we cut a hole in the battery case cover so it could still be used when the adapter is in place, but I don't have a picture of that. The MP3 player seems to have disappeared into his room...
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