How do I enable MP3 preview?

With gnome-sushi installed and then from Nautilus ....

To preview a song left click on the file then press the space bar. Gnome-Sushi will then play the file in a separate window.

Not as simple as the old mouse hover preview, but also better in that you can move the mouse away and it will keep playing until you stop it. If you click on another song the playback will switch to the new selection within the same preview window.

There is also Gloobus for preview. Just follow the steps below and you do not even need to restart Nautilus or do anything. Install PPA, Update, install packages, done.

sudo add-apt-repository ppa:gloobus-dev/gloobus-preview
sudo apt-get update
sudo apt-get install gloobus-preview gloobus-sushi

Gloobus is an alternative to Sushi (Which I find much better since it supports a couple of friendly options) and you can preview Avi files, MKV files, Video files in general, sound files in general, text, pdf, etc... Just press space and PREVIEW. Press space again and preview off. Also you can start previwing a file and use your keyboard arrows to move to another file for preview.

If you are using 13.04+ the following is the install command line:

sudo apt-get install gloobus-preview

To my knowledge this feature is no longer available in Nautilus (since Nautilus 3.0). You need to use gnome-sushi to preview audio files now.

just install gloobus-preview and don't forget to install ubuntu-restricted-extras

install gloobus-preview:

sudo add-apt-repository ppa:gloobus-dev/gloobus-preview
sudo apt-get update
sudo apt-get install gloobus-preview gloobus-sushi libgdk-pixbuf2.0-dev

install ubuntu restricted extras

sudo apt-get install ubuntu-restricted-extras

that will work for you