How to change the vertical axis from Volt to appear in dB [Pa] or SPL?

The default vertical axis labels are determined from the calibration. You have to tell WinMLS that you are using a microphone.

To do this, click the  button on the standard toolbar.

In the dialog box, make sure the correct transducer (microphone) is selected as shown in the figure below.

Then the vertical axis label should displayed in Pascal or SPL.