Hi, Mark!
Thanks for stopping by the Apple Support Communities. From your post, I understand that you are having trouble adjusting the alert volume on your iMac. Every time you change it, it seems to go back to being muted/turned to 0. I'd be happy to help!
First, try resetting the computer's NVRAM as outlined here: How to reset NVRAM on your Mac - Apple Support. Then, test the alert volume adjustment.
If it still does not work as expected, try changing it in safe mode: Use safe mode to isolate issues with your Mac - Apple Support.
Should you find the issue continues after testing in safe mode, try in a new user account: How to test an issue in another user account on your Mac - Apple Support.
Have a wonderful day.

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View notifications in Notification Center

To open Notification Center, click in the upper-right corner of your screen, or swipe left with two fingers from the right edge of your trackpad.

To view notifications that you missed, such as calendar alerts or FaceTime calls, click Notifications at the top of Notification Center.
To open a notification in the app that sent it, click the notification. The notification is then removed from Notification Center.
To remove notifications without opening them, click the the clear button next to a notification or group of notifications.
To reveal controls for Do Not Disturb and Night Shift, scroll up (or swipe down) in Notification Center.
To open Notifications preferences, click the gear button in the lower-right corner of Notification Center.

Customize alerts or turn them off

Choose Apple menu  > System Preferences, then click Notifications.
Select an app in the left column.
To change the notification style, select Banners or Alerts. Or select None.

You can also control other features here, such as whether notifications appear on your lock screen or include a preview. A preview contains a portion of the email, chat, or other content associated with the notification. You can choose to show previews never, always, or only when you unlock your Mac. /djay-pro-free-mac-download.html.

To allow or disallow notifications for specific websites, or to prevent websites from asking for permission to send notifications, learn how to customize website notifications in Safari. Download cities skylines mac torrent.

Mac Notification Sound

Use Do Not Disturb

Get Iphone Notifications On Mac

Do Not Disturb silences incoming calls and notifications. It automatically turns on when your Mac is connected to a TV or projector.

To choose when Do Not Disturb turns on and off, choose Apple menu  > System Preferences, then click Notifications. Do Not Disturb settings are at the top of the sidebar:

Learn More

Apple Mac Notification Sound Download Windows 7

For more information about Notifications, click in Notifications preferences.