If you didn’t check the ‘Enable Restricted Formats’ box while installing Ubuntu 17.10 on your machine, you’ll have to manually install multimedia codecs to make the default Rhythmbox music player and Totem video player play audio / video files.
Due to legal reasons Ubuntu can not play MP3, MP4, and other media files out of the box. You can manually install commonly used media codecs via a single package called ubuntu-restricted-extras.
Simply open terminal via Ctrl+Alt+T keyboard shortcuts, or by searching “terminal” from application launcher. When it opens, run command:
sudo apt update && sudo apt install ubuntu-restricted-extras
Type your password (no visual feedback while typing) when it prompts and hit Enter
The commands automatically install all required multimedia codecs for use in Rhythmbox and Totem, along with some fonts for Ubuntu.