AllToMP3 is available on Ubuntu, Debian, elementary OS and Linux Mint.

For Ubuntu < 16.04 (Xenial), Debian < 9 (Stretch), elementary OS and Linux Mint, your first have to install pathlib:
sudo pip install pathlib

Then, to install AllToMP3:
curl -s https://packagecloud.io/install/repositories/AllToMP3/alltomp3/script.deb.sh | sudo bash
sudo apt-get install alltomp3

You can then launch it via alltomp3 or from your distribution launcher.

Note: on old distributions, the default version of ffmpeg might not support mp3. You may have to install a more recent version of it.