I found a post from Nov. 2019 with the same issue and at the time there was no way to fix your problem with any settings in the Plex app or on the Firestick. You may have to re-encode the audio stream outside of Plex to get it to play correctly.
https://forums.plex.tv/t/no-dialogue-voices-just-background-sound-and-music/483872/2
