Hey, a fun project for a winter day at home. I found a horn at an antique shop. I unscrewed the small end and removed the reed, the thing that makes sound when air is forced into the horn. I screwed the end back on and sculpted a transition from the horn to the phone's speaker out of polymeric clay. Then I baked it for 15 minutes at 275 F.
Know what else you can use to amplify a phone speaker? Set it in a bowl.