Sounds, do they matter?

[Torax]

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[Jackolantern]

1. HTML5 is the only option that has a hope of being cross-platform since Flash has been knocked-out of mobile browsers. SoundJS is a great library for adding sounds. However, there are issues with mobile since HTML5 audio is not implemented well across all mobile browsers at the moment. Expect a lot of hair-pulling to get it working flawlessly across all mobile platforms, if that is even possible, although I would hope SoundJS could help with this.
2. All sounds must be preloaded before being played, so < 3G mobile connections and dial-up would simply have a longer loading screen. Loading sounds as they are needed will cause the game to be a mess, with sounds playing a second or longer after they are triggered or not at all. Therefore on-demand sound loading is a big no-no
3. I believe at the moment MP3 is the best supported file type for the audio tag.
4. Yes, it can be done quite easily in the Javascript. You would just put the calls to play the sounds inside an IF statement that checks if sound is turned on.

[Torax]

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[Jackolantern]

I have honestly quit worrying about IE7, and really to a decent extent even IE8. IE6 is ancient history now that it is under 1%. IE7 is on the quick train out of town. Obviously IE8 still has a pretty large share, however, you always should consider your audience rather than browser market share data. Gamers tend to be more technology-savvy, tend to opt away from IE to either Chrome, Safari or Firefox, and they tend to use newer computers with more up-to-date OS. If a game is going to be played with a browser, I don't think it is the worst thing in the world to tell someone they may get a sub-optimal experience on their current browser while providing them links to upgrade to something else. This is a very different situation than if you are trying to create an e-commerce website or create a site for a client where obviously you are dealing with a larger demographic.

Obviously it is a little cavalier to simply forget about a browser with a decently large market share. However, I see no problem at all with gracefully downgrading the experience on old browsers. You could use client-detection techniques to tell if your game is being run on IE8, and simply automatically turn sound off if it is. We don't have the benefit of having huge teams working on our games, and the time is simply better spent elsewhere outside of trying to make HTML5 enhancements run on pre-HTML5 browsers

[Torax]

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[Jackolantern]

IE10 really isn't that bad. It has a stellar ACID standards test score now, and has great support for HTML5. Its Javascript engine has gotten a massive update, even though it still lags a bit behind.

[Torax]

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[Jackolantern]

My personal opinion is that in any browser game sound should probably default to off. It can be very irritating or jolting to stumble across a website that suddenly starts playing sounds, and to avoid that the setting needs to be off to start. But the question then becomes how many people will seek out sound and turn it on? The idea of sound effects is an excellent aspect of immersion to players who are already invested in the game experience, but the issues of being user-friendly to uninvested visitors coming across your site may paint an idea of a silent game before they even know sound is available.

[Torax]

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[Jackolantern]

No problem!

What you could do is have an animation pop-up a couple of times in the play field telling them sound is available if they want it, and allowing them to click "Don't remind me again" if they prefer to play with it off. If you add sound, at least make sure everyone knows it is an option