A: Through the power line.
The full write-up and video is on Hackster. Check out the project on Hackster.io
This is what the power signature looks like when brewing tea. It's pretty clear when the power usage jumps straight up. That's the resistive heating element in the base kicking on. You see it turn off then the basket motor turn on. After a few minutes you see a tiny rectangular blip. That's the motor raising the tea basket back up after brewing. This is what we trigger off to play the WAV file. I captured the power usage with this rig I borrowed from work. It helps when you work at a electrical power monitoring company and are friendly with the support engineers.
This was the prototype circuit. This was the AC power + transducer setup. It's pretty easy to make. You just take a 15 Amp appliance cable and cut it, slip the transducer on the hot line and re-crimp the lines.