[xibernetik]

It's not really solving the "real" problem... If I'm just mashing two audio files together, that's going to be different than someone talking in the middle of a train platform and there will likely be algorithmicly-determinable difference from the artificially generated words and the naturally generated noise.

All of this aside, removing background noise is not a huge issue anymore. We have pretty decent noise-cancellation technology. Speech recognition - the other big component - has advanced a lot in recent times and is actually pretty good, although not for every company/product.

Even if it would be helpful, you'd have to record an incredible amount of noise in the first place, seeing as you're getting millions of hits a day and if you have a small sample set, the attackers will just figure out the solutions to that sample set and be done.

I'm not saying it's impossible, but I am saying it's probably not worth it at this point. Captchas (in their traditional forms) don't make sense as a long-term strategy anyways.