said by Jason Levine :

I wouldn't worry about this just yet. What details there are about this indicate that there is a conversion done on the PC that reduces the sound to a 4KB "fingerprint" and that the fingerprint can't be reverse engineered into the original sound. (Kind of like MD5 hashing the audio data.) The fingerprint is transmitted and the ads are shown based on that. Given Google's past performance, this program would likely be strictly voluntary and easy to get out of once you've opted in.

They might not even have a specific product in mind. This could just be one of those projects that the Google engineers use their "20% time" ( »en.wikipedia.org/wiki/Google#.22···.22_time ) working on.