No the newer chip should sound better, but it should be by a very small amount. The quality of the sound is more largely dependent on the quality of the speakers and the quality of the amplifier in the speakers, 99% of computer speakers are junk quality.
I'm going to guess that you installed windows 7 and when installing it windows used its own built in driver instead of the one from Realtek. I don't know if this problem has been resolved or not but a lot of people were having trouble with it. Anyway, what people were doing is before installing the OS they were disabling the audio in the MB's BIOS, then installing the OS, then installing the Realtek HD driver, then going back into the BIOS and enabling the audio.
I should hope that this has been fixed with the latest Realtek audio drivers (downloaded from realtek directly).