Yes Samsung opted for a Yamaha DAC in the SGSII, and the audiophiles, including supercurio the popular audio developer, over at XDA have been pretty vocal about their disappointment. Here are some quotes from supercurio, with some bolds for emphasis:
Based on GSMArena measurements, it seems if I do some audio hacks for Galaxy S II, I'll call it.. "audio hacks" but not Voodoo sound. IMD+Noise levels measured here are anything but good, its far worse than the stock Galaxy S badly configured. And the HP amp measured, looks .. really bad. You can forget about the shape of the frequency response, its a GSMArena measurement procedure flaw, not an equalizer or something (i'll confirm that later). I'll make other measurements and listening tests after receiving mine in a about a week but. this Yamaha audio hardware seems it's just not playing in the same league as Wolfson's.
Well no development from me about Galaxy S II audio. Maybe other people will try a few things.
This hardware is too limited for me, both in its performance and room for improvement − more or less inexistent.
I hope that reporting the full details about Galaxy S II audio will motivate Samsung to take it more seriously for the next device. They were very aware about Voodoo sound because I talked & exchanged mails with the audio engineers in Korea, and also Voodoo sound is very popular here.
I saw they did many things correctly in their software implementation, re-using some of the principles I use but unfortunately something went wrong in the process, I think probably in the hardware part on Yamaha's side. They're not so experienced building mobile codecs.
So, assuming Samsung is using the same basic hardware in Sprint's variant, if you're an audiophile, you're probably going to be disappointed with the audio quality on this device. Most users will probably think it's just fine however. If you're an audio geek, you may want to think about your purchase decision if you use your phone for a grand audio experience.