I would expect you to need two wires from the clock to the Art Controller: A common (ground) connection, and a signal from the buzzer output to the Art Controller’s trigger input. With a single wire (no ground connection), it may trigger, but it may not do so reliably. Essentially what you are doing is using the fact that it is a changing (“AC coupled”) signal, rather than its absolute value to create a trigger.
If you wish to power the clock from the 5V power supply on the Art Controller, then you need to run not just a 5V wire but also a common (ground) wire between the two. And, depending on how much current the clock requires, the regulator on the Art Controller may or may not be sufficient to power it. Since the regulator handles 150 mA and the Art Controller requires 95 mA, that only leaves 55 mA available to power the clock kit– you may want to look at how much power it draws when running on its own power supply.
Depending on your application, it may be a better choice to use the power supply that came with the clock kit, and use it to power the Art Controller, rather than doing it the other way around.
You will not be able to measure the buzzer voltage on your multimeter because it is changing very quickly. However, what you should look for is whether one side or the signal goes low when the buzzer sounds. Without looking at the code, my guess is that neither side of the buzzer goes low when the buzzer sounds. You might be able to fix the problem by intentionally AC coupling the buzzer output to the Art Controller’s trigger input, by connecting one up to the other through a capacitor.
