Sound is an underestimated medium in user experience. Think about it; your facebook tab is open and you are in the next room. Someone gives you a message and a sound plays signifying the message.
Now that’s sound user experience.
Where can I use it?
The applications are everywhere but don’t go overboard. Place an option to decrease or better, mute the sound. Also, play quality notification sounds, not something like ringtones or such crap.
Some recommended uses.
Who says you can only use sounds in apps?
In battlefield 3, the game starts minimized because loading takes time. Till then you are free to browse. When the game loads, it automatically maximizes.
The developers could add a sound because you may have gone for a pee break or have to attend a phone call.
Even in dota 2, the game could detect if it is minimized and play a sound if the match starts telling the player to maximize the game.
Anyways, sound is an unexplored territory in user experience, especially on the web. It is interesting what the developers make of it now that we have powerful apis to play with.