Most phones have 4 volume types (media, ringer, alarm, and call volume) - make sure you haven't accidentally muted or reduced the one that controls the shutter sound, which is very likely your media volume (same one for music). Also make sure the entire phone hasn't been muted, put on silent, put on vibrate, etc. Almost certainly this is the issue as I don't think there is a shutter sound option in the DJI app - if there is, that would be the other obvious thing to check.