Because your users are performing an action that could have potentially dangerous consequences, waiting until the app is closed and sending a push notification might be too late. I think, if possible, you need two steps:
- When the user performs the gesture to kill the app, ask for confirmation. I don't know if this is technically possible, though it's a pattern we see in operating systems and web pages. You could quickly warn that hearing loss might occur if the app is closed.
- Send a push immediately after the app closes with a notice. "[App] is no longer running. This may affect your hearing." And then show a button that allows them to re-open the app. This also follows patterns that are used in system notification designs.
