How can I use my Xperia X10 mini pro as an external speaker? 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.

There are numerous ways to do this like using DLNA or any of the number of apps that serve media over wifi or 3G.