iOS: aiff quiet on ipad 2 and 3

Question

Three buttons triggering three AVAudioPlayers, identical code, identical sound files except for some pitch shifting.

All three sounds loaded thus:

NSString *pathDigitClick = [[NSBundle mainBundle] pathForResource:@"tickLoVol" ofType:@"aiff"];
_playerDigit=[[AVAudioPlayer alloc] initWithContentsOfURL:[NSURL fileURLWithPath:pathDigitClick] error:NULL];
[_playerDigit prepareToPlay];

And played so:

 [_playerDigit play];

All three the same volume on iPad1, as they should be, but one of the aiffs sounds much quieter on the iPads 2 and 3.

I solved it by using another aiff, but I'd still like to know how it happens that the problem only occurs on the newer iPads.