In the sound test if you enter this combination:

FM no.46, PCM no. 12

A message in Japanese will show up with creepy sonic heads in the background. The message translates to "Fun is infinite with Sega Enterprises. - Madin".

The direct translation for the word at the end is Majin, but the context of the word makes it more likely to be spoken as Madin.
Contributed by lasertaco