I have an Xperia X10 mini pro. How can I use it as an external speaker?
I don't know of any way to make your phone a remote speaker alone but there are methods that would achieve basically the same result. Without knowing your exact goal it's hard to give you a specific recommendation but here is one way to have audio stored on a seperate device play through you phones speaker/headphones:
Setting up a wireless media server will allow you to stream media from your computer to you phone. So while it's not simply acting as a speaker, you are able to hear media that is stored on your computer on your phone.
I just installed AirBubble which is seen, in iTunes, as an external speaker. Works great.
The headphone jack has 3 channels. 1 mono mic and 1 stereo outputs. While you could modify a headset cord to input the mono channel from another set of headphone cables this would not produce any level of quality sound. I have a Xperia X10 and the frequencies on the mic input are narrow.
Highly recommend you copy the music from the mp3 player to a computer and copy from the computer to your phone. Add a larger micro SD card if necessary.