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