If you want to change or turn off your IM sounds, first bring up your AIM Preferences window by clicking the preferences icon in the Buddy List toolbar

or by right clicking your system tray AIM icon.

Now select the "IM/Chat" category on the left hand side, and then you will see the IM/Chat options displayed on the right.

Click the "Sounds" button to open your IM sound options.

Once you are here, you can change your IM sounds, or turn them off.  Use the drop down boxes to choose from the list of default sounds, and use the "Preview" button to hear them.  If you want to use a custom sound, click the "Browse..." button and find the sound file on your computer.  Uncheck the "Play a sound..." box from any sound you wish to turn off.  When you have finished, click "OK".  Your new sound changes will now take effect.