ffmpeg -hwaccel vaapi -hwaccel_output_format vaapi -hwaccel_device /dev/dri/renderD128 -i "old.mkv" -acodec flac -compression_level 8 -ac 2 -c:v hevc_vaapi "new.mkv"
Under the -acodec flag, it is what I had in another note to convert audio to flac lossless audio, max compression without loss, and down mix to stereo. the -c:v flag (synonym for -vcodec). is to use hardware accelerated h264 (HEVC) codeic for Intel drivers/GPU.
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