I think sound is a dramatically underutilized way to communicate with the user in modern software tools. Using sound for a notification is just annoying, but using other properties (pitch, timbre, length, etc) or producing varying drone sounds across time presents new ways to passively inform users of things. A classic example of this is fan sounds alerting users that their computer is under heavy load, and perhaps something is going wrong. As modern fans get quieter, computer programs should offer similar interfaces to provide all kinds of information.