If you find that your iPhone sounds are too loud, there is a workaround. Normally using the volume control buttons only adjust the ringer volume and has no affect on the “system sounds”. So if the internal volume is too high alerts will be annoying – for instance when you receive a text message alert or unlock the iPhone.
To fix it, open the iPod or Music app (formally iPod before iOS 5) and play some music through the speaker. No matter what the volume is currently set at, make a small adjustment. The system volume will be set to this new setting.
Must be a bug or an oversight in Apple’s design, because as far as I know there is no other way to adjust the internal sounds volume. Perhaps some game you were playing boosted the sound (Bad developer! Bad!)
Update (May 1, 2020). Surprisingly this is my most popular post. Apple added “Change with Buttons” to the Sounds & Haptics in the Settings app. That should adjust the volume of your alerts sound volume.