I finally identified where was the problem: the Windows 10 notifications icon (in the very right bottom corner of the desktop) was in position "Do not disturb" (that you can activate and deactivate by right-clicking).
Apparently, Opera is totally linked with it (which is not the case for Chrome or Firefox).
I did not even know that there were a "mute" option on this !
Now that my problem is solved, I would like one more time to thank you all for your kind help !