I see that question is an old one and already has an answer but I've decided to contribute rephrasing the answer. We have 2 approachs here:
Whether you are just claiming that what you listen to from someone is certain somehow, INFORMALLY, you may say: "sounds good!" because you agree with him/her.
Person 1: "We'll leave tomorrow morning."
Person 2: "Sounds good."
Whether you are questioning someone about a thing, you may INFORMALLY use "sound good?" because that is the contraction for "DOES THAT sound good?".
Person 1: "We'll leave tomorrow morning. Sound good?"
Person 2: "Yeah, sounds good."
In other words, both of them are correct if you are going to use them in the right context.