I used a track sample of my favourite axe-god Slash to help perform an informal comparison on sound playback on the iPad and iPad 2.
Both iPads had the same screen settings – and befitting Slash – the volume for both cranked up to the highest level possible.
In the linked video, the original iPad is on the left, and the iPad 2 on the right. My opinion is that sound quality is better on the iPad 2 and slightly louder, and that colours appear richer on the iPad 2 as well.
Click this link for a video of both iPads playing the track concurrently.