It's a zero-byte file. Skinners are supposed to use these instead of silent audio files in order to save space. It won't cause any issues in-game, and it will make the whistle sound silent (as intended).
Blizzie wrote:First off, excellent and clean skin. Big thanks to you for making this.
I have one issue though. Am I the only one with a corrupted "normal-sliderwhistle.wav" file? I redownloaded and it is still corrupted.