Google might have restricted their Sound Search widget to the Android Jelly Belly update; third party developers have released the installer for the Ice Cream Sandwich edition as well.
Google Sound Search (codenamed Google Ears) is a song recognition service similar to Shazam and SoundHound.
You turn it on when a song is playing in your surrounding and the app would detect the song and let you find it on the Google Play Store.
It is unlikely that Google would support it on Android ICS officially. If you really want to get it on your ICS device, you would have to rely on third party sources like this.