The simple transformation of a Midi file into Mp3 is possible either locally by using sounds fonts to improve the sounds, or externally at the audio output of all the synthesizers or in free service on the sites of the accordionists who propose in addition to their partitions, the Mp3 files.
For my part, I do not see the interest to put Mp3 on the site of partitions-accordeon.com and ditto for the Midi. With Google they can be found quite easily.