Will someone have an example of how to open the file folders of the cell phone to search for mp3 files and make it play according to the path obtained?
Much depends on where those files are. If they are correctly stored in a system shared folder, Taifun's File extension will find them. You can put them in a List View or List Picker and then choose the one to play (or set up the Player to play the whole list).
Since MP3 files are media files, they can be found / listed on all Android versions from all locations / paths of the internal and external storage (including a removable micro SDcard) and then played.
Of course, this statement only applies to the app in question with regard to its own app-specific directory (ASD). In other words, MP3s cannot be played from other apps' ASDs (since only the respective app itself has access to its own ASD).
See also here: Some basics on Android storage system