After 3.0 update i noticed that my iPhone volume was very low, even when it was set to maximum on the settings. So i did a little googling and found this sweet app.
Volume Boost can be installed from cydia and requires that you have WinterBoard installed on your iphone, so that means you need a jailbroken iphone. It supports firmware versions 2.2, 2.2.1 & 3.0.
To install the Volume Boost, open cydia and search for “volume boost”
From the search results select “Volume Boost” and install it
When installation completes, load WinterBoard and and tap Volume Boost and press the Home button to save your changes.
Thats it! 
This app increased the volume on my phone, but not that much. Hope this works on your phone and let me know what happens
enjoi
Tags: apple computers, cydia, iphone, mobile, volume boost
Volume low huh? How did you notice? built in speaker or even when using headphones?
yea, only on on the built in speaker, the sound is pretty good with the headphones
i drive truck for a living, and as you may or may not know it is very noisy in the cab wether driving or just idling in traffic. ty for posting this app.
after installing it i have no problems hearing on my iphone 3g anymore. it works great.