There was a report on the forum that some audio files cannot be played from the app. We were given access to the module on the real site and I could identify the following set-up:

• Audio files were added to my private files (size usually less than 2MB)
• Those files were added to the book as an alias/shortcut (5 files)
• They are added to the content as links (5 links)
• The filter 'Multimedia plugins' is enabled

When previewing the content in the app:

• Some audio files are displayed as an HTML5 player
• Some audio files are displayed as link
  • Those two last ones, the only thing else I can notice is that the file names contain underscores
  • On the live site they are displayed with the Flash player
• The audio files displayed as links can be opened and played
• The audio files in the media player are not played

I am guessing that mm-external-content does not account for content that was converted to a player. We should either support it, or disable the HTML5 player(s).

Reported here: https://moodle.org/mod/forum/discuss.php?d=317695