[i2shar]

Why not just use Bluetooth? I'd be suspicious if the Dishwasher app requested permission to access the phone's microphone

[colechristensen]

You can implement an audio modem with much dumber hardware and it would be cheaper and less vulnerable to nonsense, especially if all you're sending is a few bytes. Then you also don't need to do FCC certification. Seriously bitbanging an audio modem to broadcast error codes from a $0.20 BOM microcontroller and a little buzzer speaker would be a fun project to give to a summer intern. (If anyone wants to believe a highly falsified resume and would believe I'm 15 years younger, I'd be happy to join your company for the summer :D <sadly not really> )

[floating-io]

If your hardware has a clock >32Khz, you need FCC certification. It doesn't matter if it deliberately uses radio or not, last I checked.