The recording-people-dilemma (in the US)
Your app is, in its core functionality, very much the same as a dictaphone or wiretap: it records people in its surroundings. Now, when is recording a person legal? That depends on where the person using the app is, and what law applies there. Let me grab two examples, one NewYork (I learned about its law from the cohen hearing) and your choice, Massachusetts.
In New York, if a single party consents to record a conversation or phone call, all is legit. It might not be ethical or nice, but it is legal.
Massachusetts is a two party consent state, so unless both sides (recording and recorded) agree, it is illegal to record a conversation.
So, depending on where someone is, the use of a recording app can be a crime.
But is there a way out? Well...
First of all, it is not illegal to track where your own phone is. So far, one thing your app might do: tell where the phone is and save the geodata of your item. Geodata of phones has lead to quite some lost or stolen phones to be recovered.
Police is usually allowed to bypass the consent of the other party if they have a valid order, but they use the built-in functions of the phone and the service providers help to wiretap usually. Atop that, it can be quite illegal to install an application without the consent and knowledge of the user (State of Texas vs. Sony BGM).
So, the use of the software to secure voice evidence as a civil person is at worst illegal.