It's part of the admin plugin. If there is any possibility, you'd have to modify the sound file INSIDE the actual plug-in, then re-upload to the server box. I'm not sure if anyone's got any knowledge in messing around with plugin data...
you also hear it whenever your crosshair comes across a spray, and it keeps playing for the time it stays on the spray. I think its been brought up before and Prop told us to quit whining. Good luck fixing it.