There is a new jailbreak app available called Volume Booster 4.0 that supposedly boosts up the volume of your iPhone. I say “supposedly” because it didn’t work for me, although I tried on 2 different iPhones. Maybe you’ll be more successful than I was…
After downloading Volume Booster from Cydia, you have to go an activate it in WinterBoard, just like you would activate a theme. You won’t see any icon or options in the settings.app.
The app description in Cydia says that this only works with iOS 4+, but again, it didn’t work for me. I could really take some volume boost though, as the iPhone speaker is close to being useless. Anyways.
Give it a try and tell us if it works for you.