The PIC has been programmed to read a value from eeprom, increase it by one, rollover back to zero once a certain value is reached, and use this value to decide which sound to play.
In doing this, we can get our alarm clock to play a different sound each morning!
The original alarm clock mechanism has been kept the same - when the alarm time is reached, the clock mechanism pulls a line low. We use this as the ground for our board, so that it is switched on an off by the alarm clock mechanism (the mechanism keeps the line low for about 15 minutes after passing the alarm time). By doing this, we're conserving the batteries, since the entire circuit is only every powered for about 30 minutes each day (once for 15 mins in the morning, once in the evening, unless the alarm on/off switch has been moved to disconnect the ground rail from the circuit).