When all the bits of the counter are 0, the buzzer should start and continue for a second or so. The CLK_OUT from the last BCD counter is 0 when every bit is 0. Therefore we can trigger the BUZZER circuit with the final CLK_OUT from the counter. Note that the buzzer will artificially trigger when the user is cycling through DIGIT#3 from 0->9. 

The CLK_OUT signal will then be routed to the circuit shown below. 

Basically we turn the rising edge of the square wave into a pulse and use that to trigger a monostable oscillator which then enables the astable oscillator. The speaker is then driven with the output of the astable oscillator (just a square wave, no fancy waveforms here).