I've been scouring the Construct boards looking for info on audio performance for iOS web-based apps and everything seems to be pointing to no / spotty audio in this case.
You can do audio in iOS web based apps no problem. The key is to use the Sound Sprite concept and compile all your sound effects into a single audio file.
With the first user interaction (usually "Tap to Start") load the audio and immediately pause it. Then use seekTo() to jump around to the sounds that you want to play within that file. You are still limited to one sound at a time but it works.