If you want to stretch/slice/dice/etc the aspect ratio, use Plex Media Player, PMP. Download from the Announcement thread.
To set aspect ratio, change into TV Mode (in upper right, click on box with “TV” inside).
Select your username → settings → video → video aspect.
You’ll have to use arrow keys for some navigation.
See support docs linked below for more details.
FYI, PMP can also passthrough audio, if you’ve a multi-channel sound system.
To change back to Desktop Mode (looks similar to Plex Web) from TV Mode, select the computer/NAS icon (where TV icon was before). No idea if aspect ratio changes are honored in Desktop Mode. Pretty sure audio passthrough settings are honored in desktop mode.
https://support.plex.tv/articles/categories/player-apps-platforms/plex-media-player/
