You can disable this sound in Opera one but it is not that straight forward and might be undone with an update but I am not sure if it will.
Follow these steps:
- Press the windows key
- Search Opera and right click
- Open the file directory, do this twice to go to the Opera files
- While being in this folder search for: splash
- Delete or move the following file to another folder: opera_gx_splash
Good luck, hope this also works for you guys!
Bob