I must admit, I`ve posted over the Arduino forums as well and there have been some recommendations to switch a microphone but I find it odd that the 4-5 watch timers that are DIY (but none off Arduino) are all using piezo.
That said, after 10 minutes looking at the linked page - I`m still none the wiser what it is other than looking like a phone microphone and headset .... and how this would link to a digital input on an Arduino!?