اصلا اشکال نداره که error هم نمیده فقط label خالی است
privatevoid button3_Click(object sender, EventArgs e)
{
SoundPlayer sp = newSoundPlayer(@"F:\Programming\GhazalNama\GhazalNama\GhazalNama\ Resources\WAV\4.wav");
Stopwatch mywatch = newStopwatch();
if (button3.Text == "Start")
{

mywatch.Start();
button3.Text = "Stop";
sp.Play();


}
elseif (button3.Text == "Stop") {
mywatch.Stop();
sp.Stop();
button3.Text = "Start";
double TimeSpent = mywatch.ElapsedMilliseconds / 100D;
label2.Text = string.Format("Elapsed time :{0} Seconds", TimeSpent.ToString("#,##.##"));

}