Resolved, the new version 0.4.5 of Wildmidi that I compiled, required an external configuration file called "Wildmidi.cfg" which includes the list of musical instruments and the path where to find them.
The old version 0.2.3.5 found on AROS One and Aros-archive, also worked without the "Wildmidi.cfg" file and as instruments used those contained in the famous "Timidity" folder also used by other programs that make use of midi files.
At the moment the best quality is given by the old version 0.2.3.5 of Wildmidi, because it uses the excellent strments contained in the "Timidity".
With the new version 0.4.5 of Wildmidi, at the moment I have tested it with the instruments included in Wildmidi 0.4.5 for MorpOS, but in my opinion they do not have the same quality as "Timidity"
I will do a bit of research to see if there are better instruments for this new version, also because it has been fixed and now it can play a list of midi in seguence, without running into errors as was the case with the old version.