ffmpeg -i input.mkv -c:v libx265 -crf 23 output.mkv
If selecting video and audio is important for us, then we call first the code below to find out on what positions there is video and specific audio.
ffmpeg -i input.mkv
After we find out positions, we declares them (stream in 0:0 is video, stream in 0:1 is audio)
for %%f in (*.mkv) do ( ffmpeg -i "%%f" -c:v libx265 -crf 23 -map 0:0 -map 0:1 "compressed-%%f.mp4" )