Larry Jones
To set the mode to "sound on sound," you can press "P" on your computer keyboard to open the Preferences dialog, then go down to Project|Record|Recording Mode. You'll see three mode choices: Comping, Overwrite and Sound on Sound. Select Sound on Sound, then click OK. Now it should work the way you want it to.
Sound on sound is NOT the default mode...
That is the OP's answer. When comping mode was introduced in X3, the default Sound on Sound behavior was changed to comping mode. You can also right click on the record button to shift modes.