I think Opera forces the startup animation and sound when it updates. Perhaps it's thinking there's a new update when there isn't.

Download Opera_GX_91.0.4516.72_Setup_x64.exe from https://ftp.opera.com/pub/opera_gx/91.0.4516.72/win/.

Goto the URL opera://about, take note of the "install" path, and close Opera.

Uninstall Opera but don't choose to delete your data when uninstalling.

In Windows Explorer (File Explorer) (with "hidden items" and "file extensions" enabled under "View -> Show"), check if the install folder still exists. If it does, right-click it and delete it.

Then, in Windows Explorer, browse into the "C:\Windows\temp" folder, hit ctrl + a to select everything in the temp folder, and hit del to delete the selection. Skip anything Windows won't let you delete. Repeat for files and folders in "C:\Users\yourusername\AppData\Local\temp".

Clearing the 2 temp folders and the install folder should get rid of any installer and updater issues.

Then, launch the Opera GX installer you downloaded, click "options", make sure all the options are how you want them, and install.

Then, in Opera GX at the URL opera://settings/opera-gx, make sure "Run when my computer starts" is not enabled. Also, check that "Splash screen on startup" is still disabled. Then, close Opera GX.

Then, open the Windows task manager and switch to the "Startup" tab. Make sure there's no "launcher.exe" for Opera GX listed there. If there is, disable it.

Then, restart your computer and launch Opera GX. If the animation and sound happens again, close Opera GX and then start Opera back up again without restarting Windows to test if it happens again or not. That will at least tell you if it's only after a restart of Windows.

If it's still doing it, on the "Startup" tab in the Windows task manager, try disabling the Opera Assistant. Then, restart Windows and see if it goes way.

If none of that helps, you can rename your Opera profile folder and disable the startup animation and test things out to see if it stays off (at least till a new update). If that does fix the issue, then the issue is with something in your Opera profile. When done testing, you can close Opera GX, delete the new profile and cache folders that were created, rename your old ones back, start Opera GX, goto the URL opera://settings/reset, reset your Opera GX settings, restart Opera GX again, disable the animation again if needed, and see if it stays off until the next update.