Try trashing the preference file. Make sure SmartSound isn't running, go to the Preferences folder in your system folder, find SmartSound prefs, and drag it to the trash. Then rerun SmartSound. You may have to reenter the serial number. Sometimes the pref file gets corrupted, so a fresh one is needed.
If this doesn't do the trick, try rebooting with a base set of extensions. Go to Apple menu>Control Panels> Extensions Manager and select "Mac OS 9.x Base" from the pop-down menu on top of the window. Then restart the mac. If SmartSound works, then it may be a third-party extension causing the conflict.