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