There is no export to an audio file in only one case: Franz Schubert “Where to?” Maybe someone will get the export of audio. Exporting other music was successful. Link: Dropbox - Шуберт Ф. Куда?.dorico - Simplify your life
Maybe there’s a limitation with the question mark character… Try changing the file name.
I tried the option without a question mark - the result is negative. 
Sibelius midi file opens in Doriko and in this case export of audio from Doriko succeeds. But this is real hemorrhoids!
Your project exports OK for me, kafkindom. Can you try exporting to a different folder, in case that somehow makes a difference?
My Doriko does not save audio files in Apple icloud! This only happens with audio. Audio export to the desktop is successful. This happened only with this piece of music.