Some details from the application:
An obvious example of how this could be used would be if it were raining heavily in your area, Google might pick up on the sound of the rain. They could then serve up ads for say, trench coats or galoshes.
The patent application goes further than just audible cues for ads. It says that it could use image signals or video signals. In other words, it could key off your videos or images in order to serve you relevant ads.
A patent application doesn't necessarily mean a product will ever be created using it, even if the patent is granted. This sounds so intrusive, we hope that's the case, but since Google's revenue relies on ads, we can see why they would want a patent granted.