What is the Virtualizer sound effect in music players and how does it work?
In the description of PlayerPro in Google Play this was mentioned: 


  STEREO WIDENING effect (virtualizer)


and a quick search on Wikipedia about what Stereo Widening is:


  Stereo widening
  
  Widening of the stereo image can be achieved by manipulating the
  relationship of the center signal M and the side signal S: M = (L +
  R)/2; S = (L - R)/2. A positive part of the side signal S is now fed
  into the left channel and a part with its phase inverted to the right
  channel. Some boomboxes feature such a process.
  
  Another way of looking at this same effect, without extrapolating a
  center and side signal from the left and right signals, is to simply
  add the left signal, slightly attenuated and phase inverted, into the
  right channel and vice-versa. Taking this a step further, a small
  delay (20-100ms) can be added to the inverted signal before mixing it
  back in to the original for output, adding a slight reverberation to
  the effect.