In my experience, almost all low volume complaints are actually not damaged components but rather clogged membranes. The screen-like membrane on the front cover that keeps contaminants out of the phone gets clogged with oils, dirt and such and simply blocks the sound waves. A thorough cleaning with alcohol has worked many times for me.

A really high quality packing tape over the entire screen will usually be enough to use the suction cup on a shattered screen.