The subtitle supported are related to external file not to the embedded. Indeed if MyBook would be able before streaming the video to combine the two (video + subtitle) this post wouldn’t exixts.
Bottom line MyBookLive that extract from the file the video and audio part seems not able to do the same for the subtitle part.
No standard DLNA server does that. Only transcoding servers can do that, and very few NASes support transcoding DLNA, and most that do still don’t merge or split files “on the fly.”
All standard DLNA servers do are SERVE FILES – they don’t split / combine / extract or do anything else to the file contents… they all serve the file As-Is.
In order for external subtitles to work via DLNA, several things have to happen.
When the client browses or requests the video, the server checks the videos and the library to see if there are external subtitle files available. If there are, the server tells the client through an extended DLNA attribute that there’s a subtitle file available if the client wants it. It does the same thing for cover-art, etc.
Then the Client requests BOTH the video AND the subtitle stream as separate files and streams them both simultaneously.
Not only that, but the exchange of information has to occur in way that BOTH the Client AND the Server understand each other.
Now whether YOUR version of Twonky hands back the “RES” (Resource) records for the subtitle stream or not, I don’t know.
Supposedly that came with Twonky 6.0.39 and later.
Also, whether or not your DLNA Client will interpret the RES record and request the subtitle stream, I don’t know that either.
That’s one of the worst things of DLNA is that the standard is so vague, it’s subject to interpretation by all sides, and the “certification tests” done by dlna.org website are very basic.
There’s several posts in this forum about how to load newer versions of Twonky to see if it works for you.