I don't think it necessarily enhances your hearing as much as it makes you more aware of what's going on... I think maybe you just focus a lot better on one thing.

I have this theory about weed, see if you back me up on it: When you're sober, your mind s able to handle several things at the same time, but you don't really focus on one thing. When you're high, it's a lot harde to think of several things at the same time, however you can focus much more intensly on one thing. So, if you're focused on your hearing, you'll be able to pick out more sounds in your environment than when you'r sober...

It's just a theory