The easiest way to do it is to right-click the app's icon on the Dock, then click the "Quit" button. The Santa hat should be gone now. If it isn't, check whether the "Allow automatic icon changes" ...

You can play MP3 files on an iPhone using four methods: the Files app for direct playback, Apple Music to sync files from a Mac, third-party apps like VLC for more features, and cloud storage services ...