This directory contains the files that are not part of js-synthesizer, especially libfluidsynth library from fluidsynth-emscripten project. fluidsynth-emscripten and FluidSynth, the base of fluidsynth-emscripten, are licensed under GNU Lesser General Public License (v2.1).
There are two JS files for the same version (>= 2.3.0). The -libsndfile file is built with libsndfile library, which enables to load Soundfile version 3 (.sf3) files. If you want to use SF3 files, use libfluidsynth-x.x.x-with-libsndfile.js instead of libfluidsynth-x.x.x.js.