in case anyone has the same problem:
There's a ~100ms sound-lag when you use Construct2 with VMWare (maybe in other virtualization-tools, too).
In my case that's VMWare Fusion with Windows 7 on a Mac.
Not only with C2-games, but with all HTML5-Games I tested, so it's not a bug in C2, just a side-effect of virtualization. When I test the games on OSX or on a native windows-machine it works fine.