Audio Sync and  Remux is an app that enables the user to adjust audio delay of previously rendered MP4 files.  There is also an option to replace the audio stream of the original file with the audio stream from a replacement file.  It is useful for correcting the characteristic 44 millisecond audio delay (actually early) of files rendered using FFmpeg with such apps as Handbrake and the Voukoder renderer inside of Vegas.  There is no re-rendering.  Both the video stream and audio stream are copied, thus ensuring no loss of quality.