When you are changing the volume, we suspect you are doing it with the remote. This changes the volume for the “input” e.g. your computer but not the notifications itself.
To change the volume of the notifications, you should do this in Android itself using the screen and the settings menu.
On the panel, you need to swipe up and then choose the settings icon (5th from left)
Then, change the volume on the screen as shown in the picture. This will lower the notification volume. It will also save them so when you reboot, you should not have to do it again.