The audio preview is muted by default in Webcam mode.
In RTMP mode, it's possible to change the volume or mute the audio completely.
This only affects the preview in the Loola studio and not the output stream.
When hovering the mouse over the preview area in RTMP mode, control bar will show at the bottom of the preview:
Click the Enlarge button to put the player into full-screen mode:
Once in full-screen mode, change the volume:
Click the Windows button to exit full-screen mode: