Why won't Premiere 'reconnect' all the audio

I am working on a feature documentary. Every time I shut down the system (iMac using Premiere CC 2015.4). I seem to run into problems. So most of the time I leave the project running. But after this weekend, I re-booted the project and 20-30 clips play with video but no audio. They are, obviously, not off-line. The files play cleanly with audio if I open them in QuickTime.

I have deleted the cache in the Common folder, but it has no effect--the same files still play without audio. I cannot even see a common element that connects these particular files, although many are MPEG-4s--it seems almost arbitrary. When I trashed the cache, I left this file, MediaCoreQTCodecRulesCC 2015, and a folder named PTX in the Common folder--don't know if this had an impact.

I have lost a lot of time in the past few weeks with problems--deadlines are beginning to loom. Any assistance gratefully received!

GK