These files are .lsdsng tunes.

The gameboy files packs :
world I
world II
world III
world IV
the cheat code
other songs

They are meant to be played on a gameboy using a tracker called "Little sound DJ".

You can merge several .lsdsng into one .sav using this tool

A table with some of the song tempi keys notes etc.

Some of the tunes were recorded and included in these albums :