Begin with "Play Movie" highlighted and press your Left button to highlight the big "18" on the left side. Then, press your Enter button and a screen will come up explaining that most of the hidden features are audio recordings from the dubbing sessions during post-production of The Producers. By clicking on the other numbers and letters around the edge of the main menu (which looks like a theater ticket), five other audio clips can be accessed.
The seventh hidden feature is reached by highlighting "Play Movie" and then pressing your Up button to highlight "Springtime for Hitler". Press your Enter button and you will see and hear what appears to be a discarded menu animation of Lee Meredith doing her dance.
All easter eggs were generously submitted by SirDonk