Based on a user feedback at the FR Wikiconvention 2016.
In order to generate sound for books on Wikisource, some instruments should be added to the extension (ideally all, to include all cases). Apparently, there is some instruments missing.
Practical case with no missing instruments, a case with missing instruments can be found.
@Trizek-WMF, could you provide an example where some instruments are missing? The listing is part of the MIDI standard, and both TiMidity++ and Lilypond includes them, without any supplementary files.
Maybe instruments were not available at that time.
I can't give you more details about it, because the only user know who use that extension passed away since the bug has been filled. He told me that some instruments were missing.
the example he gave me requires the following instruments: flute, oboe, clarinet, horn, bassoon, timpani, violin, viola and cello.
I can't do more, sorry.