Use ffmpeg to fix mp4 files with no sound on mobile device

command

# 1. cd to your directory containing mp4 files first
# 2. will scan for mp4 files in that directory
# 3. will output new .mp4 file in parent directory with same name as original

for i in *.mp4; do ffmpeg -i "$i" -c:v copy -c:a libfdk_aac -b:a 384k ../"$i"; done;