Blow up a building!

I have two videos - one has a simple AVI with sound, and another is a silent video (MP4). Both are B&W. I need a portion of the silent video's background made transparent and then merged on top of the AVI, keeping the sound. You can then export the new, combined movie and send it to me. For anyone even the least bit familiar with After Effects, this should be pretty straightforward. Both videos are small, and low-res is actually beneficial for this project to add realism. This needs to be kept simple; no reason to go to extraordinary lengths. We're testing a concept here. Super urgent project! Thanks for your prompt response.