520181 Amend Audio File

I have made a home movie and managed to rip the audio file into a seperate WAV file. The video/audio is of my new son. I am wanting to replace the audio with some instrumental music but where ever my son cries, i would like this to be heard by the listener. So fade out the instrumental music and let the crying sound come through. THe length of the wav file needs to remain as it is. The audio file is about 30 mins long currently and i also require some good instrumental music if you have it that can be looped. Need this project done in one day. The file is about 300MB and can be delivered via dropbox.