There are three additional sliders that populate in the Settings>Sound page when a call is in progress and disappear again when the call ends. Two of those additional sliders are identified with a handset icon and the word “Calls” to the left of the slider. They are located just below the SystemSounds slider near the top of the page.
Those two sliders control the mic and speaker volume for the duration of the call, overriding any setting of the sliders lower down the page. You can only access them by quickly switching from the Calls page to the Sound Settings page when the call has connected.
I do not know what the third temporary slider does. (It is actually the first, reading from the top of the page.) It has an icon that I can not decipher.
There is no mention of these specifically Calls-active volume sliders in https://docs.puri.sm/Librem_5.html