"Sounds good" is going to be correct in the vast majority of cases, although "sound good" can be used in questions. Here are some examples: Person 1: "We'll leave tomorrow morning." Person 2: "Sounds good." Person 1: "We'll leave tomorrow morning. Sound good?" Person 2: "Yeah, sounds good." Just to comment on Ludvig's answer, nobody would ever shorten "they sound good" to "sound good."

Most often "sounds good" would be correct. I guess you most often it is short for "it sounds good" or "that sounds good". if it is not third person singular "sound good" could be correct as well. For instance short for "they sound good" or "you sound good". But in this case I guess it´s mainly about what sounds most natural and then I would definitely say "sounds good".

Romeirao, thank you for the answer :)

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: 1) Affirmative 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. ex: Person 1: "We'll leave tomorrow morning." Person 2: "Sounds good." 2) Interrogative Whether you are questioning someone about a thing, you may INFORMALLY use "sound good?" because that is the contraction for "DOES THAT sound good?". ex: 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.

sound good is correct