I installed Goodwriter, which was identified as being compatible with mp3 files (above) but it does not cause the Open in menu to appear. Is there a way to get goodwriter to open the mp3 file?
This whole situation is bizarre. I should be able to select and open the app that I want and then point it at the data file that I want it to open. Instead, it seems I have to rely on the OS to "broker" a marriage deal wherein it pre-judges the suitability of the union.

. Clearly a squishy design concept...
IMHO, files belong in a File System, where any app can get them...