[DaveBlake]

@HomerJau thanks for the example files. While Mp3tag happily shows the tags, the ffmpeg tag parser is not returning the ALBUM tag. Actually it is a mess - Mp3tag does not show everything that ffmpeg parses as a tag, ffmpeg fails to returns some tags that Mp3tag shows, and Kodi does not use some of the values that it could (it isn't expecting ffmpeg to return them). 

In a bit more detail. From the tags in the example files the ffmpeg parser (AVDictionaryEntry creation) read:

"encoder"
"creation_time"
"GENRE"
"COMMENT"
"STYLE"
"ARTIST"
"ARTISTSORT"
"CATALOGID"
"COUNTRY"
"DISC TITLE"
"TITLE"
"DISCOGS_RELEASE_ID"
"ORIGINAL_MEDIA_TYPE"
"MUSICBRAINZ_ALBUMID"
"MUSICBRAINZ_ARTISTID"
"MUSICBRAINZ_TRACKID"
"PUBLISHER"
"track"
"WWW"
"DATE_RECORDED"
"language"
"BPS-eng"
"DURATION-eng"
"NUMBER_OF_FRAMES-eng"
"NUMBER_OF_BYTES-eng"
"_STATISTICS_WRITING_APP-eng"
"_STATISTICS_WRITING_DATE_UTC-eng"
"_STATISTICS_TAGS-eng"

Mp3tag does not show DATE_RECORDED and below from that list.
But Mp3tag does show ALBUM, ALBUMARTIST, DISCNUMBER and YEAR tag values, which ffmpeg does not return.

Of  all those returned Kodi is only using ARTIST, TITLE, TRACK and GENRE (not STYLE, COMMENT and mbid tags which would be used from other tag formats). What Kodi expects to be returned is taken from ffmpeg code https://github.com/xbmc/FFmpeg/blob/mast...d3v2.c#L43,  if ffmpeg will return more tags than that  then it is easy to add to Kodi for that once it is clear what ffmpeg returns.

The main issue is the missing ALBUM,  ALBUMARTIST and YEAR values. The fault may lie with Mp3tag adding that tag in the wrong way, or in ffmpeg for not reading it, but it is not something that can be fixed in Kodi itself.

Is there a Matroska tagging standard to refer to? In ffmpeg it reads like it is using ID3v2, but the key names it returned are not e.g. PUBLISHER not TPUB. Anyway I'm afraid I don't have time to poke around ffmpeg code (Kodi is enough!) but it is open source so maybe have a look yourself or raise the question on their forums. Tell me what ffmpeg returns and I'll handle the Kodi side of it.

As for the gapless/passthrough issues, I hope we can attract the attention of a player expert. The example files will help, and maybe rsie an issue on Git