missing "system sounds"

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http://forum.xda-developers.com – [Rooted UCLB3 with Zen's Infusion A kernel, voodoo sound installed] When I go to Settings / Sound / Volume I see 4 sliders (Incoming Call Volume, Media Volume, System Volume, Notification Volume). For 3 of the 4, when I move the slider I hear a sample of the volume as expected. But for the system volume slider, when I move it I hear nothing at all. Doesn't matter where I move it, all I hear is silence. What is system volume used for? I think maybe it is used for notification of calendar items. I also noticed I haven't been getting any audible notifications for calendar appointments lately. (HowTos)