Are you on a Mac?
I have had this exact same issue a few times with nested sequences since running Sierra 10.12.4 with PP 2015.4
Yesterday the audio played, but is was out by about 30 frames. When I open the nested sequence it played fine!
Deleting media cache file, resetting Premiere preferences, turning off audio plug ins didn’t fix the issue.
The only thing that worked for me was to render out the sequence and drop it back in.