iPhone playing sounds with no apps open

When you remove an app from the app switcher you aren’t closing the app permanently; you are only removing it from the app queue and destroying any transient data in the app. It is generally not a good thing to do→Do not close "background" apps | Communities.

After you remove an app from the app switcher, if it receives a notification or an update “event” (such as new weather data) the app is relaunched to process the notification, but, since you did not open it yourself it won’t appear in the app switcher. It still runs, however, and takes whatever notification action is enabled in Settings/Notifications, and that can include notification sounds.

For more detail, I’m going to try to dig into what the App Switcher is. This is based on my knowledge of operating systems in general and how virtual memory works; it probably is not 100% accurate, but should be close. 

The App Switcher does not contain apps. Essentially it is just a list of apps that have been used. I’ll call them “stubs”. Each stub contains the ID of the app, and the last state the app was in when it last ran. If the app has a code segment in RAM it also contains a pointer to that code segment. But the stub is very small, probably just a few hundred bytes (if that much). When you relaunch an app either from the Home screen or the app switcher the list is consulted, and if the app is in the list either the code segment is activated so it can resume where it left off, or if it is swapped out, it is reloaded so it can resume where it left off.

If the app is not in the list the app is started from scratch; its entry code segment is loaded, and it must run its initialization code, open its resources and then go on to its working code. This takes longer than just activating it from the app switcher, and uses more energy than just activating it from the app switcher, so for this reason killing apps unnecessarily uses more energy than just leaving them in the app switcher list. Probably not a lot more, but some.