Running ffmpeg in browser - options?

5 years later, actually you can run ffmpeg inside the browser.

Check out https://github.com/ffmpegwasm/ffmpeg.wasm

"ffmpeg.wasm is a pure Webassembly / Javascript port of FFmpeg. It enables video & audio record, convert and stream right inside browsers."

You are going to have a tough time running something like that directly in the browser - and even if you did it would require a lot of code. I would suggest building a small web application that lives on a server that will take a media file as a parameter along with any configuration options that you want to pass along with it and let ffmpeg do the hard work on the server without having to recreate the wheel then return the modified media file. I've done something similar and it works great and in the end is a very small, lightweight application.