The only way to reliably produce .nbs audio files for Minecraft would be to use Minecraft Note Block Studio that can process MIDI songs (.mid, .midi) and convert them to .nbs file useable in the game.
Convert MIDI-sequention sound to Minecraft Note Block Studio audio.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert midi to nbs on Windows
Music editor of Minecraft music
Convert from midi file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.