How  to select  default app for any file (e.g mp3)?
1. First open any mp3 file from music  folder.
2. Now, menu pops  out showing application that can play this file.
3. Mark ‘Use default for this action’  and  select the app ( like poweramp) to make  it  a default  app for that  file. 

How to remove default app for any file ?
1. Select  setting  icon.
2. Go to  application manager.
3. Select app that was previously  selected as default  app for any  file.
4. Now, select button ‘clear defaults’.

Here is short video to illustrate above steps.
[youtube video=yD1dqw5qBgs]