Is it possible to convert a self-powered speaker into a passive one? (and if yes, how?)
My scenario: I have an old pair of PC speakers i'd like to use with my portable MP3 player even if there is no power outlet nearby...
Passive speakers like the Sony SRS-P7 are cheap headphone type speakers combined with acoustic amplification. A carefully designed sound channel amplifies the audio. Like cupping your hands to make your yelling louder, or sticking a cellphone in a bowl (I use a roll of blue painters tape, it really works well). There's a limit to what can be done based on the size of the enclosure. Active speakers on the other hand, are typically directly facing the outside.
To modify an active speaker into a passive one, would need new speakers and some manufacturing of a sound channel (Using a router or plastic injection moulding or 3d printer, etc)
The best thing you can do would replace the power and amplifier circuits with a battery powered one. And the speakers themselves will need to be resized. Essentially, the only thing you would keep is the audio cable and the case. Not practical or pragmatic.
Hit a dollar store. Buy a portable mp3 player speaker case, or a set of passive speakers...
Most headphone amplifiers are designed to drive at least 8 Ohm load. The speakers can have different resistance, but 4 and 8 Ohm speakers are quite widespread, and 2, 16, 60 Ohm or even more are rare.
If your speakers' resistance is at least 8 Ohm, it's possible to use them with your MP3 player just like headphones. The use of this application is another question - the volume will be so low that you will need to hold the speakers close to your ears, just like headphones :)
I have an old pair of PC speakers i'd like to use with my portable MP3 player even if there is no power outlet nearby...
You could build a battery-powered power supply. This could be a few AA batteries or lithium-ion battery with a regulator and a separate charging circuit.
There is likely to be spare space inside at least one of the enclosures (though this might affect accoustics).