Open demuxers for external audio files. Update the SelectionStream. Adapt all relevant places where m_pDemuxer is used; in particular ReadPacket(...) and add ReadExternalAudioPacket(...)
Adapt DVDFileInfo so that it reads infos also from external audio files (e.g. language and channels).
Only apply Seek and SetSpeed to the actually active external demuxer to speed things up.
Fix seeking chapters while having an ext. audio track selected.
(External) audio tracks don't have chapters, so we have to use SeekTime().
Set the external audio stream language from its filename if the language isn't set already.
Adapt DVDFileInfo so that it reads the language from external subtitles files.
Add VideoPlayer.AudioLanguage infolabel that returns full language name of current audio track, "unknown" localized string or empty string if no audio track is available or video isn't playing currently.
Add language recognition for external audio files to DVDFileInfo.