Wine is an implementation of the Windows API on top of X and Unix. It does not require Microsoft Windows, but can use native Windows DLLs if they are available. It provides both a development toolkit for porting Windows source code to Unix as well as a program loader, allowing many unmodified Windows programs to run on x86-based Unixes.
Mpg123 is a fast, free and portable MPEG audio player and decoder library mainly for Unix (but also ported to other platforms). It supports MPEG 1.0/2.0/2.5 layers 1, 2, and 3, with CD quality playback even on relatively slow CPUs.