Triple expanding foam can be very helpful if done properly, and horrible if misused. If you want to help sound deaden, first stuff small trashbags between panels, then use the foam to fill those. (make sure you dont over-fill the bags, as the foam expands.) also remember to tie the bags so the foam doesnt seep out. This can be a cheap and effective method of sound dampening, plus its cheap. If you are looking for a product like dynamat, check out this link:
www.b-quiet.com its less expensive.
have fun