Choose your MKV and a sound track.
An MKV can hold several sound tracks, such as two languages or a commentary. The list shows each one's language, format and channels, and the MP4 gets the one you pick.
SOLIPSIUM Super resolution audio engine
Free video tool
Convert an MKV video to MP4, with the picture copied as it is.
Your browser copies the picture from your MKV into an MP4 without re-encoding it, and keeps the sound track you pick. The MP4 is saved on your device.
Drop your MKV video here
It is converted in your browser. The video is not uploaded.
MKV or WebM, with H.264, HEVC, AV1 or VP9 picture.
Convert your video
Your MP4
Your first 20 seconds
After signing up, import the video into the editor: it repairs the whole soundtrack and exports the video with its picture kept.
What Solipsium hears
Make a free account to repair the whole soundtrack
Then import the video into the editor. Scans and short repairs are free.
How it works
From an MKV to an MP4.
MKV and MP4 are two ways of packing the same kinds of picture and sound, so most videos move across without being decoded. The page says what will happen to each track before it writes anything.
The picture is copied.
H.264, HEVC, AV1 and VP9 picture moves into the MP4 packet for packet. VP8, which few MP4 players play, is re-encoded as H.264, or as VP9 where this browser has no H.264 encoder.
The sound is copied or converted.
Sound an MP4 holds, such as AAC, Opus, MP3, FLAC or AC-3, is copied as it is. Vorbis is converted to AAC, or to Opus where this browser has no AAC encoder. Sound an MP4 cannot hold and this browser cannot decode is left out, and the page says so before you convert.
See what the sound needs.
The quick check listens to the first five minutes of the track you picked for clipping, dropouts, hum, background noise and muffled sound. You can then hear its first 20 seconds repaired, with no account. Solipsium's free try is built for speech.
Questions
Before you choose a video.
Which sound track goes into the MP4?
The one you pick from the list. The first sound track in the file is picked to start with. The others are left out.
What happens to subtitles?
They are left out of the MP4, and so are chapters. Keep the MKV if you need the subtitles, chapters or other sound tracks.
Will AC-3 or DTS sound work?
AC-3, E-AC-3 and DTS are copied into the MP4 as they are. A browser that cannot decode them plays the MP4 silent, and the page says so before you convert; a player that reads them plays the sound.
Does converting lose quality?
Not when the picture is copied: the MP4 holds the same packets. VP8 picture is re-encoded with the H.264 setting the video enhancer uses, and loses a little detail. Converted sound is compressed again.
How large is the MP4?
About the size of the MKV, since the picture and sound are the same streams. Leaving out other sound tracks and subtitles makes it a little smaller.
How long a video can it take?
Any length, when your browser writes straight to a file, which Chrome and Edge do. Other browsers write the MP4 into their storage for this site, or into memory, which holds about 200 MB. A VP8 re-encode takes videos up to 1920 x 1080, 60 frames a second and 30 minutes, in ordinary 8-bit colour.
Is my video uploaded?
No. Your browser reads the MKV and writes the MP4 on your device. Only if you press Repair the first 20 seconds does any sound go to Solipsium, never the picture, and Solipsium keeps only its first 20 seconds and deletes them within an hour.
Make an old recording easier to hear.
Solipsium works on voicemails, voice memos, answering machine tapes and camcorder audio.
Try it on a recording
The check and the 20-second try need no account.