I have no experience with this platform. Try to force the app to perform transcoding.
This can be achieved by either
- restricting the allowed bandwidth to below of what the original file has
- disabling DIRECT PLAY and DIRECT STREAM
Both settings are to be made in the Preferences section of the app on your Chromebook.
Or simply try to use the web app https://app.plex.tv which should automatically trigger transcoding.
See Subtitle settings - #5 by OttoKerner
and perhaps also Adding a second audio stream manually - #2 by OttoKerner