Every time I open Opera up, my computer makes a funny buzzing sound. It seems related to my Microsoft trackball mouse, because moving the trackball is the only way to temporarily stop it. At first I thought it was the whole computer, but when I shut down Opera getting ready to reboot, the sound went away. I opened Opera again, the sound came back. I closed it again, the sound went away.
I would start by going to Opera preferences (alt-p) then go to advances->notifications and turn off opera sounds. For grins - go down to the opera Voice options on the advanced prefs screen and then turn off opera voice.
That should turn off all the opera noise making capability.