The sum level is more or less the output volume of all objects played back by a sound card. Every output channel has a sum level control and a master control in addition to it, which controls all outputs jointly . As the highest level is zero dB, this value should not be exceeded. Otherwise, sound information may get lost or even sound errors may creep up.
In the Project Explorer, click on tab 
Audio level. There is an audio level display and a control (... small triangles) for every output channel. Normally, there are two for stereo sound, but some projects with multi-channel sound may also have distinctly more than that (e.g. for surround sound, multi-language shows, etc.). In addition to this you will find the Master Control on the right (double triangle) which affects all outputs equally.
Please note that the volume of objects on asynchronous audio tracks cannot be changed using the audio level controls. See also Asynchronous audio playback.
You can adjust the sum level by grabbing and moving one of the little triangles with the mouse. This works even during playback. By double-clicking a triangle, the control can be set back to zero dB.
The peak level is indicated by small markers on the dB scale. Reset them by clicking on the audio level bars with the mouse. The peak level must be no means exceed zero dB. When the K-System has been activated the peak bars turn yellow once zero dB have been exceeded. See also The K-System - Checking volume and dynamics.
Per default, the sum level controls are grouped into stereo pairs. Below you can see how grouping can be changed so as to allow joint or independent setting:
All controls: Press the Shift key while adjusting them. This causes all controls to be adjusted to the same value.
Individual controls: Press the Ctrl key while adjusting them.
Please see also