>have an app on the phone that plays a series of tones on the speaker, which the lock receives and uses as an audio key The series would have to be time-depend

>have an app on the phone that plays a series of tones on the speaker, which the lock receives and uses as an audio key The series would have to be time-dependent, or you'd be vulnerable to replay attacks. Or maybe the lock plays a challenge and the phone plays the response.