the wav c# player stops playing the song after 4/5 seconds

i have made a simple wav player in c# and here is my code

 *******************************************

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Media;

namespace WindowsFormsApplication1
{

public partial class Form1 : Form
{

public Form1()
{
InitializeComponent();

}

private void next_Click(object sender, EventArgs e)
{

}

private void nnext_Click(object sender, EventArgs e)
{

}

private void playpause_Click(object sender, EventArgs e)
{
try
{
string sptbvs = sptb.Text;
SoundPlayer my_wave_file = new SoundPlayer(sptbvs);
my_wave_file.PlaySync();
}
catch (InvalidCastException)
{
MessageBox.Show("Error 502 :invalid Path.");
}
}

private void sptb_TextChanged(object sender, EventArgs e)
{
string sptbvs = sptb.Text;
}

private void before_Click(object sender, EventArgs e)
{

}

private void bbefore_Click(object sender, EventArgs e)
{

}


}
}
*********************************************

but when i click the play/pause button the song starts properly and after some seconds (usualy 4) it stops playing the song 

 why is this happening

please help