I know the Spark has no microphone but uses the phone mic to record audio; videos downloaded from Spark to phone have audio...videos copied from sd card have no audio track.  What is the purpose of using phone mic if you cannot get audio out of the device?

The cached videos (made on the phone while you fly) mix in the audio from the phone, but the video quality is often terrible (dropouts, artifacts etc) as it's live from the drone. You can download the original video from the SD card to the phone, but then the audio is lost.

It would be much nicer if the audio was mixed in to the original quality video when it transferred to the phone but I don't see any way to do that.

You can get the low quality video+audio out of the phone, just  "share" it from the app, or find it in the "DJI_RECORD" device folder in google photos (android). I suppose you could splice the audio from the low-q file to the high quality file, but anyone's guess if it would match up with the dropouts etc.

I have the same question, when i just download from sd card and watch it, suddenly feel so empty after viewing through the phone cache first.

There are apps to extract the audio from a video file, i use one called VideoToAudio pro on my iPad, i m sure there are others for droids or win.  You can then import the audio file in a video editor and add I think to your HD clip from the card.

I also use it to extract th audio from copyright free music available on you tube.

If the drone recorded noise all you would hear is the props spinning = pointless. Add some music behind your videos in post.