This is actually a long story but in short
The Galaxy S II uses Yamaha audio hardware. The Galaxy S II's predecessor, the original Galaxy S, used Wolfson's WM8994 DAC.
Galaxy SI has wolfson codec , which was open source.
Supercurio a well known developer and audio expert here in XDA was able to create good sound output using his voodoo app.
In galaxy SII samsung replaced wolfson with yamaha and this was closed source.
The sound quality for audiophiles using galaxy sII was terrible. ( not to my ears as I am a certified non-audiophile
Being closed source no further development was possible.
IN the words of supercurio "
With today’s Android audio implementation, all kind of media are sent to the DAC as a 44100 / 16bit / Stereo stream. Despite the usage of a fixed rate, Yamaha’s codec is not able to provide a very clean output.
Galaxy S II DAC output quality is limited by several kind of distortions. So far, no firmware was able to fix those despite early Korean updates describing “audio clarity improvements”.
When playing music, those artifacts are perceived as “lack of clarity”, “reduction of stereo separation”, “loss of detail” and “lifeless sound” (opposite of lively)."
The good news is that with new firmware releases many people have said that the sound quality of Galaxy SII is much better. So i guess its going to be better in GNote.
However we wont get Voodoo sounds improvements.
You can see the links below for more detail