Can iPhones play Opus files?
Update Nov 2017:
Opus is now included in iOS 11.

iOS 11 and Android Spearhead Universal Support For Opus Audio Codec, MP3’s Overdue Successor



Judging by this conversation on the Apple forums, it seems unlikely an iPhone can use the format directly.


  Since Opus is not compatible with most popular players and devices so far, it's absolutely impracticable, no matter directly play Opus files on Windows Media Player, Android, PS3, Xbox 360 and iTunes or sync Opus to iPhone, iPod, iPad via iTunes, or edit these Opus in iMovie and QuickTime.
  
  Under the circumstances, the best solution is to convert Opus to MP3 and other more compatible audio formats in order to play or edit Opus in the above popular players and devices.


One of the posters mentioned that Adapter (freeware) can convert then to a playable format, MP3, AAC etc.