To change a mixer volume, use the channel faders for that mixer, as follows:

To lock the left and right mixer channels together, select the Lock option for that mixer. When you lock the mixer channels, GNOME Volume Control synchronizes both faders.

To silence a mixer, select the Mute option for that mixer.

Any mixer that has a Rec option can be a recording source.

To specify the current recording source, select the Rec option for that mixer.