One of the interest of a smartphone (and in particular of the Android) is to customized it easily. We are going to see how to chose a MP3 file as a ring for the phone (for all contacts or one).
At the root of the SD card, create a directory "media",then a sub-directory "audio", in which 3 sub-directories must be created "ringtones" (for the rings ), "alarms" et "notifications. Copy in each of these directories the MP3 files (or any other supported format, e.g. WAV) that you may want to use.
Then go to menu Preferences ==> Sound and display, and then the files you have inserted will be available in the matching menu.
I have noted that after personalising the ring tone for one contact, the Android device must be rebooted to take it in effect: it's not great ... but that's the way it works for me !