Right now PMS lists and displays the first audio stream in any container (MP4, MKV, etc.), even if it is not flagged as the default stream.
For instance in this movie, the 4th stream is the default, enabled audio stream (DTS).

The 2 audio streams before it (AAC 2-ch and AC3 6-ch) are not enabled (not marked as default).
Media Browser recognizes the DTS stream as the default audio stream and displays it correctly in Media Browser Classic.

However, Plex displays and uses the first audio stream (in this case the 2-ch AAC stream) instead of the enabled, default stream (should be the DTS stream).

I'd like to submit a feature request to have Plex use the enabled 'default' audio stream instead of the first stream in a container.