This audio issue is the most common problem with all of the lastest apple devices as they are using the new low power audio chips by Cirrus Logic which sacrifises loudness, but maintaining the same audio quality.Try using a high end Earphones/headphones which is capable of producing better sound.There is no problem with your new ipad.
Take it back.
I took my 3rd gen back on the off chance it was faulty and it was. Now my sound is superior to the iPad 1st gen. And it is far superior to the first gen when running it through my guitar amp. My store took it back without any question or test, but you might have to get them to listen to a song on your iPad and then the same song on a new iPad.
An updated version of an iPad should not have inferior sound.
I am happy that you are enjoying the new iPad. However I did some research and found that Apple and Cirrus Logic have indeed changed the chips.
The following CNET article will give you more details on this.
Starting with iPhone 4S they are now using the lower chips, I guess even on the new pieces of iPad 2. I went to a store and heard the performance and was sad to find the same difference.
I guess this is something Apple has chose to do and we as loyal customers have no option but to live with it.
I too found that the speakers on the new iPad are good. The amplifier will give you a god sound because what the new chip does is that it keeps the quality of music under control but sacrifices the loudness which gives its oomph factor.
The amplifier gives your new ipad the much required power to the sound and its perfect. The difference is quite notable when you use just your headphones.
I've made similar observations in the following thread. I'm also able to tell differences in audio quality in different iDevices. One way to mitigate the loss of music quality is to increase your music bit rate if you haven't done so. It won't help fully though.
I tested the following devices and ranked their audio quality.
Headphone: Sennheiser CX880
.mp3 320 kilobits
They all sounded different.
Acoustic (Bass vs. Sound Clarity)
Bass Clarity 1. Nano 4 (VERY punchy and strong bass) 1. Nano 3 (crisp, clear vocal reproduction, 'hi' instruments can be heard clearly')
2. iPhone 4 & Nano 3 (Very close performance, strong bass)
2. Nano 4 (crisp, clear vocal reproduction, 'hi' instruments can be heard clearly) 3.- 3. iPhone 4 (very good, clear vocal reproduction, 'hi' instruments can be heard clearly) 4. New iPad (Strong bass, more mid than other devices) 4. New iPad (bass sounds like its on the verge of crackling, acceptable vocal reproduction)