# speed up video and audio at the same time
# 2x speed up command:
ffmpeg -i input.mkv -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]atempo=2[a]" -map "[v]" -map "[a]" output.mkv
# 1.5x speed up command:
ffmpeg -i input.mkv -filter_complex "[0:v]setpts=0.666*PTS[v];[0:a]atempo=1.5[a]" -map "[v]" -map "[a]" output.mkv
# speed up formula (x must be between 0 and 2):
ffmpeg -i input.mkv -filter_complex "[0:v]setpts=<1/x>*PTS[v];[0:a]atempo=<x>[a]" -map "[v]" -map "[a]" output.mkv