Sound detection


I wanted to build a device for a hearing impaired person and was looking to get some advice in what type of component I should use.
I used some microphone with the arduino mega 2560, but had issues with sound detection, it randomly picked up sound within approximately 10 feet.

Am open to any ideas you might have on a project which could detect sound, I should also be build in such a way that after it picks the first sound, after a given interval, it starts scanning for other sounds within a certain area.

If that was too much information. The device should just be able to alert the user when any danger is coming.

Example: a hearing impaired person is crossing the street, a really fast car is coming directly towards her, but before the car can actually make contact with her a DEVICE warned her that an object was coming and just in the blink of time she was able to move out of the way.

Thank You.