As Cocos mentioned, it is probably the initial loading of your sound file which is causing the one time delay.
In your implementation add the sound action:
SKAction *eagleSound;
In your init method add this:
eagleSound = [SKAction playSoundFileNamed:@"eagle.wav" waitForCompletion:NO];
Then whenever you need to play the sound, use this:
[self runAction:eagleSound];