How does one answer that with a typed response? Sound is subjective. Someone might say it sounds great and you might buy one and think its lame. Stereo fanatics argue audio components and speaker systems the way we debate train issues here. The best thing you can do is visit your LHS and have them run a loco with one for you. Then you can decide how it sounds to you.
Hi NPG SoundTraxx has a few sound samples that the following link. You can get an Idea what the decoder sounds like from these. Remember that the speaker in the model will be a lot smaller than the one on your computer. http://www.soundtraxx.com/products/dcc/tsunamisound.htm
