how to do audio fade out

Question

I want to fadeout audio after a perticular time for example:- my audio duration is 4 sec and fade out time is 2 sec then after 2 sec i want to fadeout(decrease the volume of player) the audio

for that i have written this function

but its not working properly audio is faded out but the duration for fade out is less than 2 sec

where in this function delay is the time of fade out

-(void)fadevolumeDown1:(NSNumber *)delay
{

     float t=self.player.duration-self.player.currentTime;
     if(t<=fadeOut)
     {
         self.player.volume=self.player.volume -0.05; 
     }

     volumeChange=YES;
     if (self.player.volume <= 0)
     {
        volumeChange=NO;
        self.player.volume=0;
        mute=NO;
        first_decrease=false;
     } 
     else
     {
         if(!goBack)
         {
            if(self.player.currentTime<=self.player.duration)
            {
               [self performSelector:@selector(fadevolumeDown1:) withObject:delay afterDelay:0.1];
            }
         }
      }   
 }