Tap an mp3 file - if the app you want as the default doesn't open, open the one that does open in Application manager and clear defaults. Then when you open an mp3 file, it will give you a choice of apps and 2 buttons - "Just once" and "Always" (or words to that effect). To make an app the default, choose it and tap Always. To not have a default, to open music in the app of your choice every time, tap Just Once.
(You can't have a default app, and also choose the app you want every time - that's mutual exclusivity.)