I use Samsung Galaxy S3 and S4. Previously I could play mp3 files with the "music" application. Now I have 10 players but the plain "music" player is gone and no builtin player can play an mp3 file from my memory card. Why? There used to be a player in the android that could do it. Instead I have google play which seems new.
In most players you need to tell it where to find the music files or have the ap scan the device. i use Music folder player its free and does the trick