How can I change the sound I hear when someone sends an IM in Yahoo Mail?

In Yahoo Mail, I can display a list of online contacts and can have conversations with them. I hear a sound when they message me but want to change it. I also have Yahoo Messenger, and can change the sounds there, but no change I make there affects the sound in yahoo Mail. Does anyone know how i can change the sound in mail? I'm not talking about when I receive emails, and I'm not talking about when I am in Yahoo Messenger. I'm interested only when I receive an instant message from one of my online contacts in Yahoo Mail. Thanks
As I mentioned, none of the changes I make in Yahoo Messenger change the sound I hear when someone messages me when signed into Yahoo Mail.