my iphone plays music every Sunday morning

One other thing that I did not think of is that there could be an Alarm set to play music once a week. Tap the Clock app and choose Alarm on the bottom. If you have any alarms turned on, then shut it off. By tapping the alarm, you can view its settings. The settings would match up to Repeat on Sunday at the time selected and the Sound option gives you the song to play.

The only way I know why this would happen is if you have an Automation set up in the Shortcuts app.

If you open the Shortcuts app and select Automation on the bottom, do you have any set up to play music at a certain time every week?