Android
Set a Custom MP3 Ringtone auf Android
Android ships with a limited set of default ringtones, and most people never look past them because the option to add a personal one is not obvious.
Custom ringtones work through the sound settings, not the file manager. Android only picks up audio files once they are registered as a ringtone inside the system, which is why simply having an MP3 on your phone is not enough on its own.
To add one, open Settings ➔ Sound & vibration ➔ Phone ringtone ➔ My sounds ➔ Add. From there, open the menu, select the directory where your MP3 is stored, and choose the audio file.
Once the file appears in the list, select it and tap Save to make it your active ringtone. Placing MP3 files directly into a folder named Ringtones on your phone storage beforehand makes them show up faster inside this menu, since Android scans that location automatically.
If you only want a special sound for one contact instead of the whole phone, use the per-contact ringtone option in your Contacts app instead of changing the system-wide setting here.