Local researchers Nirupam Roy and Romit Roy Choudhury published an interesting little study that showed that the vibration motor common in mobile devices (providing “haptic” feedback) can be used to listen to nearby speech. 
As they note, this is hardly Earth shattering in principle, in the sense that any vibrator will, in principle, respond to sound vibrations. The interesting thing is that it was simple to do, and they were able to capture speech quite completely. No special processing was needed to reconstruct recognizable speech. “Even automatic speech recognizers were able to decode the majority of the detected words and phrases.”
I think the investigators were prepared to apply significant processing and fancy algorithms at this problem, and were surprised at how little was needed.
This technique isn’t necessarily very useful (or dangerous). Most common personal devices have microphones, so there isn’t much need to use the buzzer for this purpose. I suppose that it might be a security hole if you somehow were able to block access to the microphone, but attackers could access the vibrator. But that seems like a long shot, compared to just hacking one of the numerous microphones in the environment.
Another possible use might be for voice controlled wearables, which could be designed to use one component for both haptic output and voice input. Eliminating the need for a separate microphone might be a valuable cost, weight, and power savings for some kinds of wearable devices. On the other hand, it might be even better to use this idea to design a component specifically for this role.
This is neat work, and goes to show that there is no substitute for actually trying things out on real systems. Neither theory nor simulator could really tell us whether this would work, or how easy it might me.
- Nirupam Roy and Romit Roy Choudhury, Listening through a Vibration Motor, in Proceedings of the 14th Annual International Conference on Mobile Systems, Applications, and Services. 2016, ACM: Singapore, Singapore. p. 57-69.