small audio application

Hi to all from belgium in europe ( just above France :-) I am a sound engineer for some musicians here . I have no company . I have a crazy idea for another kind of stuff . Is it possible to program an audio application that just puts reverb on the audio input of a pc or mac ? So what i'm looking for is a small part of a program like Garage band . Just be able to listen to the input mike of a mac or pc but with direct monitoring in the headphones with a simple reverb ( long ) to begin with . What would be the cost of such a soft ? Thanks for reading . Heave a nice day .