Audio is the most underused UI element
Audio would have to be the most underused UI element within a web application. In fact it's virtually never ever used! Many people I talk to about UI design don't even acknowledge audio as a UI element. They see it as "music" or that audio should NEVER be used in a web page.
I'm not refering to audio for mouseOver events in flash animiations.... I think just about everyone hates those. I'm talking about using audio for push & pull (aka polling) operations, that may occur while the application is not in focus.
Email is a great example of this. A tradional mail application like thunderbird will poll your mail account every x minutes. When it detects a new email it shows a new window pane in the lower right hand corner of your screen and 'ding'. I think this is a perfect example of when you should use audio. In fact gmail/hotmail would benefit greatly from this as a lot of people leave gmail open all day. Of course there are some people that just hate 'noise' from web pages, or they don't want to interfere with their music in the background. These are valid reasons not to, but if you get them the option everyone should be happy.
Like any UI element, it can be abused. When the web became trendy, circa 1995, people who 'programed' web pages thought it would be trendy to include anoying audio snippets when mousing over links (primarly in flash) and I guess this has stuck around in a lot of peoples minds. I hope that developers can get over this mentality and use audio in a productive way.