'NSInvalidArgumentException', reason: 'Resource beep.wav can not be loaded' iOS Sprite Kit

I'm making a game for iPad with Sprite Kit (Objective-C).

I often use the following code (for example):

[self runAction:[SKAction playSoundFileNamed:@"beep.wav" waitForCompletion:YES]];

All goes well, until after a while my app crashes and I suddenly get the following error:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Resource beep.wav can not be loaded'

I have no idea why, because the sound could be loaded en played correctly before the error..

The same error can also occur with other sounds, but always after some time ..

Any ideas?