The mixer levels can be “frozen”

If you want to measure absolute quantities, e.g. SPL or Voltage, the level calibration must be checked in Measurement->Calibration..., as shown below

If WinMLS is set to control the windows mixer, and the gain steps are not set to be defined, a warning message is given if you try to change the volume as shown below.

The warning is given because changing the mixer levels will make the level calibration invalid. In order to change the mixer level, go to Measurement->Calibration.... and either use defined gain steps, or turn off the level calibration check boxes.