Venturebeat was first to report and has more screenshots.
If you have the Google Mobile App installed on your iPhone, go to the Settings tab, scroll to the bottom and keep swiping upwards until a secret option dubbed ‘Bells and Whistles’ appears (this also works in the foreign language versions of the app, and it will stay there once you’ve found it).
The hidden menu lets you change the theme color of the app and its default sounds to chicken or monkey noises. It also enables you to turn on a ‘Live Waveform’ which visualizes speech as a graph, and an effectively useful one that lets you opt to to open hyperlinks in the app itself.
Now go tell all your friends that you found it first!