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Found it!
The setting isn't in Opera, it's in Windows.
With Windows 10 I had to go to Settings->System-Notifications & Actions
Scroll down to Opera, click on it. Set 'Play a sound when a notification arrives' to off.
Now I can listen to google play music and not get a notification interruption any time the song changes... or blasted when google calendar sends a notification.
PEACE AT LAST! 
Found it!
The setting isn't in Opera, it's in Windows.
With Windows 10 I had to go to Settings->System-Notifications & Actions
Scroll down to Opera, click on it. Set 'Play a sound when a notification arrives' to off.
Now I can listen to google play music and not get a notification interruption any time the song changes... or blasted when google calendar sends a notification.
PEACE AT LAST!
I'm running Opera 39 and I can't find the option to disable notification sounds -- just notifications entirely.
Can anyone gently hit me with the clue bat?