AppleScript to put your Mac to sleep after X minutes, fading out the system volume shortly before sleeping. Can be turned into an app via the "Export" feature of the AppleScript Editor.

sleep-timer.scpt

-- written by Florian Bender, 2015
-- forked and edited by San Kim, 2019
-- based on work by Michael Wyszomierski <https://wysz.com/wyszdom/2009/06/simple-sleep-timer-with-applescript/>
-- license: Public Domain / CC0 <http://creativecommons.org/publicdomain/zero/1.0/>
tell application "System Events"
	set fadeTime to 10 -- time in seconds for fade change
	-- display alert "Volume: " & volumeValue & ", Delta: " & fadeChange
	display dialog "Sleep time (min):" default answer "60"
	delay ((text returned of the result) * 60 - 30)
	display notification "Going to sleep in 30 sec" with title "Sleep Timer"
	delay (30 - fadeTime)
	set volumeSettings to get volume settings
	set volumeValue to (output volume of volumeSettings)
	set fadeChange to (volumeValue div fadeTime)
	repeat with X from 1 to fadeTime
		set volumeValue to (volumeValue - fadeChange)
		set volume output volume volumeValue
		delay 1
	end repeat
	sleep -- alternatively: do shell script "pmset sleepnow"
end tell

@fbender

The main change I made is moving set volumeSettings, set volumeValue, and set fadeChange down, to right before the fadeout starts.

Previously, the fade out would start at the volume level defined at script initiation. So if you run the script when the system volume is set high, and then you manually decrease the volume, the script would increase the volume to the original level before fading out.

Now, it starts the fade out at whatever level the volume is, at the time the fadeout starts.

Thanks for providing the script, it's been super helpful!