In the Master Panel, beneath the ON AIR indicator, you will find the LISTEN button.
This button opens a pop-up Live stream window, which enables you to monitor the streams that have been configured previously in the Stream Settings page on the System menu. In the Live stream window, a drop-down menu enables you to switch between the streams which are currently available.
Beneath the drop-down menu for stream selection is a rectangular pause/play button on the left, and an orange volume control bar on the right. This volume control only adjusts the output level of the pop-up Live Stream window, not the output level of the Airtime server itself. Click on the left side speaker icon to mute the output, or click on the right side speaker icon to raise output to maximum level.
To adjust output level in between muted and maximum, click on the corresponding place in the orange bar. When you have finished monitoring the streams, you can close the pop-up window in the normal way, depending on the browser you are using. In Firefox, you can close the window by clicking the X button in the top right corner. This action will not shut down the output from the Airtime server, only the stream monitoring on your desktop computer.