I think you should update your code as like below
- (IBAction)btnPlay_click:(UIButton *)sender
{
if (!self.audioPlayer)
{
NSString *audioPath = [NSString stringWithFormat:@"%@/%@",self.strBookPath, [arrMP3Files objectAtIndex:currentPage+1]];
NSURL *audioURL = [NSURL fileURLWithPath:audioPath];
NSError *error;
self.audioPlayer = [[AVAudioPlayer alloc] initWithContentsOfURL:audioURL error:&error];
[self.audioPlayer setDelegate:self];
[self.audioPlayer prepareToPlay];
}
if(![self.audioPlayer isPlaying])
{
[self.audioPlayer play];
[btn setBackgroundImage:[UIImage imageNamed:@"pause.png"] forState:UIControlStateNormal];
}
else
{
[self.audioPlayer pause];
[btn setBackgroundImage:[UIImage imageNamed:@"play.png"] forState:UIControlStateNormal];
}
}