I’d say it depends on how you think the player is likely to first encounter the game.
Personally, if someone sends me a “try this, it’s cool!” link, and when I click on it a barrage of sound effects comes out of my machine, then I’m likely to reach for the close button pretty quick. (Thing is, I’m probably at work at the time.)
On the other hand, if I’m deliberately downloading a game with the intention of playing it for a few minutes, then I want it sound effects and all.
So, OFF by default for casual/browser-based games, ON by default for anything less casual?