If you read the fine print, the next significant release will be written in Common Lisp but the current releases are written in Crystal.
It's a Common Lisp program for end users and I can literally use it in 3 clicks, thanks to its AppImage (for Linux x86-64). Good (and rare) stuff!
It's also impressive to see that the audio decoding is mostly done in CL, without bindings.
Kind of, given that Common Lisp is highly influenced by Interlisp-D, Lisp Machines, so naturally it has the necessary language features for such kind of low level coding.
Nice, last time I used this it was actually written in Crystal. I used it at the time to render VGM files, very handy and fast. Author is nice and responsive as well, I reported an odd issue with a VGM I had and she responded quickly.
It seems the developer has no plans to support MPEG-4 audio files.