ffmpeg trying to convert AVI to MP4 but not playing I'm trying to convert a video file from AVI to MP4 but it doesn't seem to play, my code:

exec("ffmpeg -i ".$video." -c:v libx264 -pix_fmt yuv420p -movflags faststart ".$withoutExt.".mp4")


Even plain code:

exec("ffmpeg -i ".$video." ".$withoutExt.".mp4")


Any ideas please and thanks in advance.
You should specify the audio codec, e.g. -c:a libvo_aacenc.