Looks like you're creating a new MediaPlayer instance to play each sound. You should either reuse them or clean them up.
From the documentation of the MediaPlayer.create() method:
Convenience method to create a MediaPlayer for a given resource id. On success, prepare() will already have been called and must not be called again.
When done with the MediaPlayer, you should call release(), to free the resources. If not released, too many MediaPlayer instances will result in an exception.