Yeah, I missed that somehow. Looking back, there is an embedded srt file there.
What do you mean the whole library? You Plex Dance individual items. You don’t use the same steps for an entire library, it won’t work.
The network issue was a guess. Different devices handle errors differently so it’s hard to compare with a working NVidia Shield… I still think it’s the audio track causing issues.
Another thing I just noticed when I wen back to look at your XML is that the audio track is index 0 and the video track is index 1. Typically, these are opposite and video is 0. I don’t know if this makes a difference, but the app on the Amazon Fire TV may be having trouble dealing with this.
Remuxing the file with the video and audio in the normal order may fix your issue.