System volume???

[madspeed]

Just wondering what the system volume is? What triggers this sound? I see it can be shut off using one of the volume control apps, but not sure what it is

 

[mountainbikermark]

Only thing it does that I've found is lock sound volume.
Mine got turned off somehow and myvolume+ app had it and I turned it back on.dadburned if I know how it got turned off as I see no system volume option anywhere in the settings menu
I use an Incredible fwiw.