We talk about the sounds from casino games and how it distracts the gambler's attention from even realizing that they've lost so much already and that's true.. little things like that usually adds up; we're talking about a cool interface and a attractive colour choice, accessibility at ease etc.
As crazy as it sounds, people extend their stay on a casino site for one of such reasons, but i doubt that it can become an uncontrollable feature to look past, especially since you could just put the sound out and stay with the budget.
As you said, turning off any sound it's pretty easy and doable in an online casino, but in a physical sounds matter a lot and you can't turn them off. I mean that different strategies are applied depending on which kind of casino we are talking about. All the casinos want players to keep coming, but online casinos don't put much effort into the sound. I always close the sound when I'm playing slots, and I put on the music that I like.