The following are two common usages of FFMPEG.
The first command extracts an audio track from a video file in the exact format that it was multiplexed info the video file.
The second example shows how you can adjust the volume of an audio file with re-encoding.
ffmpeg -i input.mpg -map 0:1 -acodec copy -vn output.ac3
ffmpeg -i input.ac3 -filter:a “volume=10dB” output2.ac3