Thanks for the FFmpeg information. The AVI file that is not working does have an extra audio stream (Stream #0:2: Audio: pcm_s16le). Please try removing this audio stream from the AVI file using one of your video tools and see if your Panasonic Viera TV will display the external subtitles now. Also, in Mezzmo, right-click on the AVI file and click 'Properties'. Go to the Transcoding tab in the Properties dialog and delete any transcoded files so that we make sure the AVI is delivered to your TV and not a transcoded file.