SoX is a cross-platform command line utility that can convert various formats of computer audio files into other formats. It can also apply various effects to these sound files, and, as an added bonus, SoX can play and record audio files on most platforms.
BatchEncoder is an audio files conversion software. It supports most of popular audio file formats including lossy and lossless compression. The program is very simple to use. BatchEncoder is basically GUI front-end for command-line tools. All it does is create background processes and pass arguments to it with options set in presets/format menu and additionally adds input and output file paths.
The classic alternative to Window Media Player. Winamp is a cross platform, extensible, free and skinnable media player with a large community and a long past. Development has stopped, but may start again, meanwhile, the development community has largely moved to Foobar2000.