Developer website: Clixsounds
Agent Audio lets you safely replace the "snd" resources of any application or file that already has them. Select the program you wish to customize to view the available sounds. Then select the sounds you wish to use, click the "move" button, and Agent' does all of the work for you! Replace the music of your games with your favorite song, customize your "new mail" sound in your email client, or change the sounds in your America Online client! You can literally customize any software with sound resources to your liking.
System 7.0 or later