While this might work, wouldn't the sound have to be audible to the user in order for this to work? And therefore when you browse to a web-site it is going to let out a bloop on LOUD just to test the device?
Personally if that happened to me I would never visit your web-site again.
And I am not saying it has to be a random noise or very loud the accelerometer on the iDevices since the iPhone 4 is superb a very low sound will probably be sufficient. The sound can be easily integrated into the natural flow of the site, as many sites already do small beeps or play music etc to provide feedback to the user.
Edit: Oops, this is a Safari question, not an iOS app question, so this doesn't apply.