There are 2 repositories under mml-compiler topic.
NES Sound Driver & Library (NSD.Lib)
JavaScriptで書かれたStandard Midi Player
MML compiler
MML Compiler SAKURA Rust
compiler of mml-like code and renderer of generated audio
WonderSwan Total Sound Driver (WTD)
MML Compiler / Text to Midi Converter
Yet another modified MML interpreter, in Rust.