Here’s a simple FFmpeg how to that will cover just a portion of the framework’s abilities. We will see how you can use the terminal to perform simple conversions of various audio file types including all popular and widely available formats. Using FFmpeg right from the terminal makes it better for performance compared to downloading and installing a GUI tool for FFmpeg, and can also offer more precise settings for the advanced users.

Full article here:
How to use FFMpeg to do simple audio conversion (Howtoforge)
Related: Combine Audio and Video with ffmpeg (David Walsh)