Every day, Phil Connors wakes up in his bed only to realize that it's Groundhog Day all over again. And how does he know so quickly? Because he's always awakened by the clock radio sitting next to his bed. The radio is always playing "I Got You Babe" by Sonny and Cher, followed by a pair of annoying radio announcers who say,
RADIO HOSTS: Okay, campers, rise and shine!
Over time, the clock radio becomes a sort of Doomsday bell, endlessly signaling to Phil that he has failed to escape the endless cycle of Groundhog Days. We can see this especially in the way Harold Ramis at one point makes the number of the clock look huge and the sound of its numbers flipping over (this was before digital clocks) echo.
Then there's the nice montage where Phil finds increasingly brutal ways of destroying the clock radio. It's not until he hears the radio announcers saying something different that he finally realizes that he has escape Groundhog Day and can move on with his (new and improved) life.