Audio signals are an important (and underrated) source of contextual information. For example, in the case of a complex, multivariable process where you want to be able to detect (subtle or sudden) changes, over time, sound can give you everything you need and still leave your eyes and hands free, e.g. the stethoscope, the Geiger counter, sonar, hard drive chatter.

I've been thinking about applications for sound lately, and it's kind of shame that there's no good way to deliver sound to a single person, except through headphones, which are obstructive and have big downsides for social interaction. (And loudspeakers, of course, are rooted in a single place; their signal is public and diffuse.)

Here's an article about sound-focusing through a technology called HSS. Two high-frequency wave signals are sent out in parallel; the different between them, mathematically, is a third signal. When (and wherever) the two signals interfere, the third sound is produced.

Breathless, but lots of suggested applications here.