You can download i686 of ffmpeg compressed file from this link.

After downloading it, extract it, then upload the ffmpeg file inside to your repl.it project.

You can also use the below shell approach to download and unzip ffmpeg binaries on your repl.

Firstly download ffmpeg binaries using wget.

wget https://johnvansickle.com/ffmpeg/releases/ffmpeg-release-i686-static.tar.xz

And then extract the archive.

tar -xf ffmpeg-release-i686-static.tar.xz

And then allow the files to read & write files on the server.

chmod +777 ./ffmpeg-release-i686-static

And that's all, now you are ready to go.

Try using

ffmpeg [command]

into the shell.