What should an operating system do? It should manage computer resources. It runs programs, controls access to hard drives and speakers and graphics, allows you to manage files, and so on.
Something I want from the operating system is volume control per application. By default, I want only the focused appliation to play sounds. If another application wants to play sounds, I'd like control over the volume.
The prime use of this, I think, would be to control your web browser's volume while you are surfing the web. I don't like most pages that play music or sounds. I especially don't like it when I have multiple pages up and they want to play their own music. I'd like to play music in a music player while surfing the web, and with only a single volume control, I can't do this.
Operating system makers: please add volume control to every window's titlebar!
Update [2006-01-21]: I just read that Windows Vista will have per-application volume control. Hooray! Unfortunately it won't be in the titlebar. It'll be up to each application to create a widget to integrate volume into their own app, and you can also control volume from a separate window.