Interestingly, it appears Apple improved on the iPhone 2G speaker with the iPhone 3G and iPhone 3GS but when it released the new iPhone 4 design the speaker was significantly quieter than the previous two devices.
The iPhone 4S and 5 were the loudest overall with the iPhone 5 being the clear winner.
● iPhone 2G: 91.0 dBA
● iPhone 3G: 95.0 dBA
● iPhone 3GS: 97.6 dBA
● iPhone 4: 92.8 dBA
● iPhone 4S: 97.9 dBA
● iPhone 5: 100.3 dBA
Take a look at the video below!