MMLX is a music programming language used to make NES Chiptunes. It extends from Music Macro Language: http://en.wikipedia.org/wiki/Music_Macro_Language
It is short for MML eXtended. Everything written in MML is valid in MMLX, but there are additional features.
If you are familiar with web programming, MMLX is to MML what SASS is to CSS
For a getting started tutorial check out:
https://github.com/ccampbell/mmlx/wiki/Getting-Started
For complete documentation:
https://github.com/ccampbell/mmlx/wiki/Documentation