[oztrained]

Is there any known disadvantage to using OMXPlayer instead of the default MMAL? 

Did you try #719? That has a fix that looked relevant.

omxplayer is deprecated. It is limited in features compared to mmal (the video player runs on gpu so doesn't benefit from new kodi features).
It has missing features including dvds with menus, alsa audio, software decode, drm/widevine.
It's mainly intended for use on Pi0/Pi1 which have much slower arm cores.   

Thanks

#719 gets me back to the same correct audio detection as #630 for Movies and TV Series. However it still has the same audio issue as #630 and previous versions had with Recorded TV from Argus PVR

1. Using MMAL the audio is now correctly decoded as DD or DTS for Movies and TV Series
2. Recorded TV from Argus PVR using MMAL plays audio as PCM (this happened even in Kodi v17 using MMAL) however it is correctly played as DD audio using OMXPlayer. This is the same in #719 playing the Argus PVR recorded TS files from the Kodi menu TV > Recordings
3. However using MMAL and selecting the same TS file using the Kodi File Browser the Argus recorded file plays with DD audio.