Hi grandma m
welcome to the Apple (user-to-user) Community.
Re: "Lullaby notification no longer playing at wind down time:
Why can't someone answer these very simple questions
why has the warning chime (lullaby) stopped sounding
and
why can I only set the wind down warning 15 minutes or more before needed?"
We are all Apple users, like yourself, in this community, so likely no-one here has any explanation for what Apple does, or why Apple does what it does.
Sorry I do not have an Apple Watch handy to check out the wind down time for you:
Wondering which Watch OS update you installed: Watch OS 9 is the newest update.
I do not know if anything below will help you to return to your previous settings ... flexibility seems to be a requirement for technology, as it is always moving ahead.
Apple support's article for WatchOS 9
Track your sleep with Apple Watch – Apple Support
says:
- Open the Sleep app
on your Apple Watch. - Tap Full Schedule
✽. Change Wind Down time: Tap Wind Down, then set the amount of time you want the Sleep Focus to be active before bedtime.
✽. The Sleep Focus turns off the watch display and turns on Do Not Disturb to
reduce distractions before your scheduled bedtime.
You can:
Adjust your wake time and bedtime:
Tap your schedule, tap Wake Up or Bedtime,
turn the Digital Crown to set a new time, then tap 
Set the alarm options: Tap your schedule, then turn Alarm off or on
and tap Sound & Haptics to choose an alarm sound.
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Change sleep options
- Open the Settings app
on your Apple Watch. - Tap Sleep, then adjust these settings:
- Turn On at Wind Down: By default, the Sleep Focus begins at the wind down time you set in the Sleep app. If you’d prefer to control the Sleep Focus manually in Control Center, turn this option off.
- Sleep Screen: Your Apple Watch display and iPhone Lock Screen are simplified to reduce distractions.
- Show Time: Show the date and time on your iPhone and Apple Watch while the Sleep Focus is active.
The complete article (from the blue link above) gives a lot more options and settings.
(You can also change these sleep options on your iPhone.
Open the Apple Watch app on your iPhone, tap My Watch, then tap Sleep.)
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More support, if needed:
You might want to reach out to the Apple Watch Support Team
(eg" online chat, phone, eMail etc)
There is a "Get Support- start now" link
(scroll to the very bottom) on their webpage (link below)
Apple Watch - Official Apple Support
_________________________________
If you wish, you can send Feedback to Apple. They will likely not reply, yet your feedback will reach those who could make changes to the software.
Web-form for feedback:
Feedback - Watch - Apple
All the best :-)