AB altitude blending is based on the tallest object in the scene (doesn't matter if it has the same material, which is a serious limitation IMO). You'll want an A-B-C blend where C is used as an alpha channel and where C is black will be texture A and white is texture B. So your C will be a graduation from black to white and where you want B to start (assumed to be the "loud" texture) is where the blend will take place. Hope that makes sense.