Most people's default music playing app on Samsung phones was Google Play, but since it was discontinued, Spotify became everyone's new default app...the solution is to just download a new music playing app!
If Spotify opens up when you tap an MP3 song when looking at your files, it must be set as "default" to open audio. To change the default app in your Samsung, we recommend you to follow the steps on this Samsung's support article. It is important to have a different app that can be set as default for MP3 files.
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The app is trash, and Spotify knows damn well they made app run as default without anyway of changing. Without Spotify my personal audio files open with a generic audio app, like it did before Spotify recently changed something. I'll never use the app again.