To convert audio files on Linux Mint you need to install the proper tools. You may choose from different applications capable of converting between the most used formats (mp3 for storing and listening, wav for creating audio disks and some others).

Please keep in mind that the bitrate is one of the most important feature of an audio file as it determines the quality of the sound. A good choice of bitrate is 320 kbps to convert some music.

GNOME users may find useful:

winff is a graphical video and audio batch converter for the command line video converter, FFMPEG. It will convert multiple files in multiple formats all at once.

SoundConverter -  application to convert audio files into other formats

KDE users may find useful:

soundkonverter - an audio converter frontend

pacpl - for the KDE4 version, install pacpl and right click in Dolphin to convert audio files

More details can be found at http://www.howtoforge.com/converting-audio-files-on-linux-mint