Selene is a GUI audio/video converter application for converting files to many multimedia formats. It can handle MP3, OGG, MP4, WEBM, and so on. Here the instructions about how to install Selene in Ubuntu via PPA. The PPA provides Selene for 12.04, 14.04, until 16.04. Thanks for teejee for providing this PPA.
sudo apt-add-repository ppa:teejee2008/ppa
sudo apt-get update
sudo apt-get install selene
Explanation: those three commands set a new repository, resync the repository setting, and the install selene. It needs to download about 18 MB data in our Ubuntu 14.04. Selene is not available in official Ubuntu repository for now, so installing it from PPA is the only way (easy way).