Sometimes, MP4 files (mostly from a GoPro), MOV files, and AVCHD (MTS) files that are imported in Premiere Pro CC 2015.3 import without audio. The files are imported as video-only. You could also get an error stating "This File has no Audio and Video Stream" while relinking files with these formats.
This issue affects Premiere Pro CC 2015.3 (versions 10.3 and 10.4).
Delete all the files from Media Cache and Media Cache Files folders. The location of these folders can be found under Preferences > Media.
By default, media cache files and the media cache database are stored in these locations:
Mac: /Users//Library/Application Support/Adobe/Common
Navigate to the Media Cache location and rename the folders to Media Cache Files_OLD and Media Cache_OLD respectively.