Passive amplifiers are wonderful for increasing the sound coming out of your phone without electricity. Take this Baritone iPhone Speaker for instance: it is an acoustic sound amplifier that you can hang on the wall. It works like a megaphone, amplifying the sound through its horn.
To get started, you just have to put your phone in the appropriate slot. InstrumentalAudio is selling this handmade item on Etsy. You can find out more about it here.