Update!
The HTML5 GameMaker's sound system has been upgraded. Gets rid of latency, makes it more responsive and allows "layering".
By layering I mean, imagine a mario game. And a sound plays as you pick up coins.
You run into a bunch of coins and you here "Dingdingdingidnidngidnigdngn". Those sounds are layered cause they play on top of each other as you pick up coins.
Before, sounds had to wait before playing again-"Ding Ding Ding Ding".
Enjoy!