What is an AVI file?
AVI, Audio Video Interleave, is a container Microsoft introduced in 1992: a box that holds a video track and an audio track side by side. What it does not tell you is what is inside. An AVI from 2004 probably holds DivX or Xvid video with MP3 audio; one from a camcorder might hold motion JPEG; one from a screen recorder might be uncompressed and hundreds of megabytes a minute. Two files with the same extension can need completely different decoders.
That is the root of every AVI problem. The container is old enough that everything can open the box, and the things inside are old enough that modern software often cannot read them.
Why AVI will not play
Browsers never supported it, so an AVI cannot go on a web page or into most social uploads. Phones usually refuse it. Windows Media Player and the Photos app play some AVIs and fail on others depending on which codec is inside, which is why the same folder can have one file that works and one that shows a black screen with sound.
VLC and mpv play nearly all of them, because they carry their own decoders. That makes them a good way to check whether a file is intact before converting it.
Converting AVI to MP4
MP4 with H.264 video and AAC audio is the format everything accepts: browsers, phones, editors, and every upload form. Converting an old AVI usually makes the file smaller as well, because modern compression is far better than what a 2004 encoder managed, and the picture is re-encoded from whatever the original held.
Quality drops slightly whenever video is re-encoded, which is unavoidable when the old codec has to be decoded and a new one written. At sensible settings the difference is invisible next to the improvement in compatibility.
Frequently asked questions
Why does my AVI have no sound?
The audio track uses a codec your player lacks, often AC-3 or an old MP3 variant. VLC will usually play it, and converting to MP4 rewrites the audio as AAC, which plays everywhere.
Is AVI better quality than MP4?
No. The container does not decide quality; the codec inside does. An old AVI at a given file size almost always looks worse than an H.264 MP4 of the same size.
Can I put an AVI on a website or Instagram?
No. Convert it to MP4 first; browsers and social platforms do not accept AVI.
Will converting lose quality?
A little, because the video is decoded and re-encoded. Keep the original AVI if it is the only copy, and convert at a high quality setting.