In Ubuntu, I want to record everything that plays through my speakers.
If I were to play a movie and a YouTube video at the same time, the tool would record the mix of both.
It should output as an .ogg or a similarly popular format. It must be free, ideally open source.
You can use ffmpeg; it can record system audio from PulseAudio (which is the default sound server on Ubuntu), the command line should be similar to this :
ffmpeg -f alsa -i pulse -c:a libvorbis output.ogg
If it isn't installed on your system for whatever reason, here's a command to install it on an Ubuntu system (should also work on Debian, enter your password if asked) :
sudo apt-get install ffmpeg
Note: I didn't test this; feel free to comment or downvote if the above command does not work.