If the tiny iPhone speakers aren't loud enough for you (they're not) and you don't want to mess with wired headphones, getting wireless speakers to connect, or trying to cup your hand over the speaker to make it sound louder, then it's time to take a technology-free and rustic approach to boost that sound.
This cool new Volta Sound Block uses a solid piece of alder wood from the Pacific Northwest and a real bison horn to naturally amplify the volume of your 4/4S/5/etc to a proper listening level, something that you would think a 21st century device could be able to do by now. Just slide it into the wool-lined slot, connect the removable magnetized bison horn to the side, and hit play. Voila! Volume.