When somebody mentions roof flashing to me I think of thin sheet metal that is easily bent into whatever shape you need. I don't see how it could be of help except for filling in over an opening so you could put something like dynamat extreme over it.

As for the Flashtape, I could see where it could be useful. One of the goals of sound deadening material is to change the weight of the exterior sheet metal so it resonates at a much lower frequency. Anything that adds mass will do this. A few layers of flashtape could be a poor mans alternative. Another goal is to weaken the energy causing the metal to vibrate. To do this the sound deadener's use a material or combination of materials that absorbs the sound energy. Usually it does it by turning it into heat. Alternating sheets of lead and felt bonded together makes an excellent sound deadener and has been used in ships engine rooms for decades if not over a century. More modern methods substitute foam for the felt, and high density vinyl for the lead.