This is the only post I will ever make on this board, but I would like to add my voice to those who have already asked for a straight forward means to disable the startup sound. Here's why:
I am very high sensory anxiety, because of reasons I don't have control over. I set my speakers to quiet time for the most part. But the problem is that you play an unexpected noise, and to make it worse you seem to want to play it at a high volume.
I'm not cool with that. What I want is a disable option. Otherwise Opera is a cool browser. But I need the noise to be off. I am sure I am not alone in this. It is an accessibility issue*, not convenience for many of us.