There's an option in your preferences to turn off the sound... just do that. It's in IM/Chat and click the Sounds button at the bottom, there you can either choose a different sound file to be played or choose to disable it completely. You can also edit all other sounds that are made by AIM. Also, if you have trouble finding which window recieved the message you can set AIM to flash the window on the taskbar when a message is recieved. I find that no sound and just visual notification is quite good.

Il papa caca nei legno?