This is a PT3 format chiptune music player. The format is kind of standard de-facto for AY-3-8910 chiptune music on Russian ZX Spectrum clones. About ten thousands songs has been made since mid-90s when it was first introduced.
The program emulates AY sound chip and plays PT3 files loaded into the SPIFFS. The sound is output through the sigma-delta modulation based DAC via the built-in speaker. It also supports i2s stereo DAC if connected (change define in the sketch, no run-time option to switch between outputs at the moment).
Programming lesson learned: do not use sigma-delta while accessing SPIFFS, it crashes the device (i.e. disable sigma-delta while accessing files).