“The video you have sent me does not work.”
Matroska file format is not supported.
H.264 video is supported, but not with the default yuvj422p color format.
MP3 sound is supported, but the sample rate 22050 S/s (and probably others) is not supported.
Suggested solutions:
Convince your client to use a better operating system.
Convince your client to install video player with non-Microsoft codecs.
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Use the following avconv command line.
avconv -i infile -ar 48000 -acodec libmp3lame -pix_fmt yuv420p -y outfile.mp4