[Phil W]

:

Yes exactly thats i would have done on a professional level. But now i
have to play with whats provided to me.
.. mp4 made of a shaky cellphone camera .. however with the courtesy of
friend here i have managed to get them as mp3 with correct speed. Next am
focusing on FFMPEG to batch convert them to mp4 with just one still pic..
getting close to it now :-) Thanx

Just a totally crazy idea:
if whatever person records with a smartphone - why not simply use a
different app for audio recording instead of making bigger files, that take
longer to send?

Tell that person to try "Audio Recorder" for Android - it´s free and open
source and pretty simple to use. Whoever can use a video recording app, will
be able to use "Audio Recorder" just as well!

At least, it would save you one step of work, since you would not need to
extract audio from mp4 files - with over 600 files, that´s a good amount of
time