SUMMARY

Ability to set certain hours as 'quiet times'.
The schedule would override current settings temporarily to turn off all or particular sound devices as specified.

Example 1:

Shara lives in a dorm/shared room/hostel or bedroom with their spouse, and doesn't want to disturb anyone between 23h and 09h.

Currently: 

Shara turns their laptop on at 23:30, and it makes noises before they can figure to mute the volume, awaking other people.

Solution:

Shara sets a quiet time schedule for 23:00 - 10:00 and sets it so only the headphones work during this period (unless they override it). Future sleepless nights or arguments are thus avoided.

Example 2:

Jones has a laptop they use for Messenger at home, with it's built-in speakers.

Currently: 

Jones gets to work one morning, opens the laptop and it un-suspends, and immediately disturbs his workmates because pings on Messenger and music from an advert on a website that was already open starts to play; all before they can mute the volume. This is disturbing but also sounds deeply unprofessional.

Solution:

With a programmable quiet-time, Jones can be sure to avoid embarrassment and annoyance at work, without having to constantly think about it or plan for it.

Example 3

Jan uses their speakers and headphones at various hours, and has no set schedule.
Jan never knows when their baby will be asleep, or their housemates.

Currently:
Every times Jan turns on or in particular un-suspends their desktop, they are anxious that some unbeknown sound will come on, before they remember to, or figure out how to, mute the speakers. This has woken the baby, and has got them in trouble with housemates who think they are inconsiderate.

Solution:
Jan uses Quiet Time to turn off the speakers automatically every time the desktop is turned on, or awaken from suspend or hibernate.
This avoid embarrassment. Jan uses the quiet time icon to turn it off when needed.