If the Plex client does not support the audio format in the movie, it will be transcoded by Plex Media Server to a supported format.
What Plex client are you using to play the movies (PC, AndroidTV, SmartTV, etc)? Make sure it is up to date.
What audio/video formats are in the movies? Attach the text output of MediaInfo or Plex’s XML analysis of the movie. See Investigate Media Information and Formats for details. Save the XML information as a text or zip file. The forum does not accept .xml attachments. Drag the file into the compose window to attach. It will insert at the cursor location.
What is shown in Plex Dashboard -> Now Playing when you are watching the movie? Is the audio or video transcoding? Attach a screenshot if possible.
Consider updating your version of Plex Media Server. Current versions are 1.19.5.3112 (public) and 1.20.0.3181 (Plex Pass / beta). 1.16.6.1592 was released in Aug 2019.
