Use Google operators to find unprotected directories of audio files or files of type .mp3.

-inurl:(htm|html|php) intitle:"index of" +"last modified" +"parent directory" +description +size +(wma|mp3) "Nickelback"

This will turn up several directory listings of Nickelback MP3’s. Replace “Nickelback” with the artist, genre or keyword of your liking.

Note: this is not intended to be used to download mp3’s illegally.

More:

• Download mp3 (music) files with wget
• Converting Audio/Videos using ffmpeg – FFMPEG examples
• How to Convert a Flash Video File (.flv) to MP4 with FFMpeg
• Geek to Live: Find free music on the web
• How-to find interesting video, sound and image files in unprotected directories