You can't - it's a windows event sound. To bypass it you could use javascript to update the hash on a url, and have another function firing every second which reads and checks it. if it has changed, it loads new content based on that.
However that would need xmlHttpRequest objects, so you may as well scrap the iFrame and update an element on the page using element.innerHTML.
However if you start updating the hash to load content, you won't get new history events added automatically like you do in an iframe. But that is if you want history events? are you aware that pressing the back button at the moment will fire history events in the iframe as well?
If you want history events to be added still, then take a look at RSH (Really Simple History) library.
updating hash: google location.hash
xmlHttpRequest: google ajax or xmlhttprequest, or take a look on w3cschools
Hope this helps. Also, take a look at that url AndyB posted. Click the links in the blog, use google. From clicking the MSDN link i got
"Quiet Navigation- (FEATURE_DISABLE_NAVIGATION_SOUNDS)- This control disables the sound that is produced when clicking on links."
Good luck