ffmpeg

That's an intimidating one. It seems ffmpeg is a must-have tool for anyone dealing with multimedia files on a somewhat regular basis. I like the way they phrase it on their website:

FFmpeg is the leading multimedia framework, able to decode, encode, transcode, mux, demux, stream, filter and play pretty much anything that humans and machines have created.

I probably use no more than 2% of its capacity. Here's what usually covers my needs:

# Trim a video from a given start time mm:ss to an end time mm2:ss2 (omit the -to flag to trim till the end):
ffmpeg -ss mm:ss -to mm2:ss2 -i path/to/input_video.mp4 -codec copy path/to/output_video.mp4

# Extract the sound from a video and save it as MP3:
ffmpeg -i path/to/video.mp4 -vn path/to/sound.mp3

It's worth having a look at the official website.