What Plex client/platform are you using?
What version?
What version of Plex Media Server?
First guess: Plex does not download audio tracks with Language = Unknown. May or may not be a bug. Not enough information to tell.
If possible, try downloading another movie with audio tracks in multiple languages that have the Language field correctly set. See if Plex downloads all audio tracks.
Alternately, edit the file to correctly set the Language field then re-analyze and refresh the metadata for the movie. If Plex does not pick up the new language setting, then Plex Dance the movie. Once Plex sees the changes, try the download again. Additional details below.
If you examine the movie with a tool such as MediaInfo, you’ll see there are several fields or flags for audio tracks. One field is Language. Another is Title.
The audio track you show in the picture has Language = Unknown and Title = “English DTS.”
If the movie uses the MKV container, you can use the Header Editor in MKVToolNix to set the Language field correctly.
If the movie uses the MP4 container, and you’ve access to a Mac, you can use Subler to set the correct language flags. If on Windows, use XMedia Recode. Tell it to copy, not convert, the video, audio, etc tracks.
MediaInfo text output of a dts audio track. Note Title and Language fields.
Audio #2
ID : 3
ID in the original source medium : 4352 (0x1100)
Format : DTS
Format/Info : Digital Theater Systems
Codec ID : A_DTS
Duration : 1 h 27 min
Bit rate mode : Constant
Bit rate : 1 509 kb/s
Channel(s) : 6 channels
Channel layout : C L R Ls Rs LFE
Sampling rate : 48.0 kHz
Frame rate : 93.750 FPS (512 SPF)
Bit depth : 24 bits
Compression mode : Lossy
Stream size : 947 MiB (4%)
Title : dts 5.1
Language : English
Default : No
Forced : No
Original source medium : Blu-ray
MKVToolNix Header Editor showing Language setting for an audio track:
