Hi Grant,
Thanks for well explaining the issue.
The AudioObject feature within Schema plugin looks for audios if inserted using the Embed Block. This is intend to be used for audios hosted somewhere else rather than your own site. (example: a SoundCloud audio url).
This is different than the “Audio Block“, where you upload your audio file on your own site.
Unfortunately; The Schema plugin does not support Audio Block. This mean audios inserted by an Audio Block will not show in structured data markup output.
Also, I’ve just tested Scheme with Meks Audio Player plugin, everything seems to work as expected.
Do you get any error in your error log file that could be relevant to this issue?