An MKV often carries several audio tracks: languages, a director's commentary, a surround mix. It can also have chapters. Choose one track and either one long MP3 or one MP3 per chapter, which is ideal for concerts and audiobooks.
Open MP4, MKV, MOV, AVI, WMV, FLV, RealMedia, DVD VOB, camcorder MTS or almost anything else, and save it as any of 32 formats. The converting happens in this tab. Your video is never uploaded.
Each file is identified from its first bytes as soon as you add it. Once the engine is loaded it is read in full, so you see the real codecs, size and length before you convert.
MPEG-4 Part 14 on the ISO base media format. The safest choice when a file has to play anywhere.
ffmpeg -i input.mp4 -map 0:v:0 -map 0:a:0? -vf "scale=trunc(iw/2)*2:trunc(ih/2)*2" -c:v libx264 -preset veryfast -crf 23 -pix_fmt yuv420p -c:a aac -b:a 128k -ac 2 -f mp4 -movflags +faststart video.mp4A 5.1 mix puts dialogue in the centre speaker, music at the sides and effects behind you. Squeezing it into stereo means deciding how much of each goes left and right. The standard recipe often buries voices under music; a dialogue boost fixes that.
MKV can hold nearly any audio codec. If the track is already MP3, we copy it out untouched in seconds. Anything else is decoded and re-encoded, and how much quality survives depends on what it was.
Four common cases and the settings that suit them. Pick one to load its choices into the picker above.
Neighbouring tools, with a typical result for each.