Nautilus plays sound files on mouse over (no need to open a media player if you just want to quickly hear how the song goes). I find this to be highly irritating behaviour, so I turned it off!

To resolve: ALT+F2 and type gconf-editor. In it, navigate to: → apps → nautilus → preferences. Change preview_sound to never.

Or if you prefer the terminal:

gconftool -s --type string /apps/nautilus/preferences/preview_sound never