Well, the second one "It sounds like a good idea" certainly sounds more natural to most English speakers and would never be disputed. I suppose "It sounds a good idea" might be considered either a bit more 'posh'...or it can just sound wrong to many anglophones.

I also have been thinking that it should be "sounds like/as". Let's wait for a native English speaker's response.

"It sounds like a good idea" is correct. The first one is wrong. Well, I think the first one 'might' be used in some dialects. I only think that just because when I read it, I imagine it with an accent (maybe something that sounds vaguely north English or possibly even Scottish; I have no idea why I'd have that association). Is there anyone from northern England or Scotland around here who can confirm or deny this?

They are the same