As mentioned on its website, FFmpeg is:

A complete, cross-platform solution to record, convert and stream audio and video.

FFmep is a powerful freeware, as mentioned on the Wikipedia page

FFmpeg includes libavcodec, an audio/video codec library used by many commercial and free software products, libavformat (Lavf),[6] an audio/video container mux and demux library, and the core ffmpeg command line program for transcoding multimedia files.

libavcodec is actually used in VLC a popular multimedia you may be familiar with. Personally I value FFmpeg for its CLI that allows many audio/video format manipulation and conversion. For instance, in order to get a sample of 3.4 seconds from a video (let’s say in .mp4) starting at 56min42sec500ms , do this:

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ffmpeg -ss 00:56:42.500 -i yourvideo.mp4 -t 3.4 -c copy out.mp4         

Also, to convert audio file in batch formats in batch:

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# wma to mp3
for i in *.wma; do ffmpeg -i "$i" -b 320kb "${i%wma}mp3"; done

# flac to mp3
for i in *.flac; do ffmpeg -i "$i"  -b 320kb "${i%flac}mp3"; done

# flac en alac
for i in *.flac; do ffmpeg -i "$i" -acodec alac  "`basename "$i" .flac`.m4a"; done;

The CLI is fairly simple and if you know a bit a bash then I,m sure many ideas pop up in your mind and make you realize how powerful this could be.