The Airport Express was reverse engineered to make a program called Shairport. This lets you stream music from a Mac or iOS device to a PC running Shairport.

Linux

Download the tar.gz, extract it, read the INSTALL.md file for instructions: There is an apt-get command to run, then you run make. You can just run ./shareport.pl and it will work. I didn't need to install the PERL modules, I guess Ubuntu already has them.

Windows

There's a Windows port called shairport4w