Installing these codecs should allow you to play most video and audio files you can find on the internet. (avi, mp3, mov, etc.)
For Ubuntu 18.04: see this
For Linux Mint 19.2:
1. Make sure you are connected to the Internet.
2. Open the "Software Center" form the main menu and search for "codecs".
3. Choose "Multimedia Codecs"
4. Press "Install"
Open a terminal and enter this command:
sudo apt install mint-meta-codecs