Technically, "good sounding piano" is correct. But I should warn you, even if it is grammatically correct, this is pretty awkward phrasing and a native speaker wouldn't say this. It would sound more natural to say something like "the piano that sounds good," or better yet, to use a word other than "good," like "in-tune."

The main difference between "good" and "well" is that "good" is usually used as an adjective while "well" is an adverb. Unlike "well", "good" can also be used as an adverb in many types of informal English. For this reason, "good" is more flexible than "well". Examples with "good" (adjective) * Today was a good day. * This is good. * Good news! * This is a good piano. Examples of good (informal adverb) * How are you? --> I don't feel very good. * You cook good. Examples of well (adverb) * How are you? --> I don't feel very well. * She sings well. * The football stadium is well designed. Pro tip: Sometimes, "well" can be used as part of a compound adjective: * The well-designed football stadium looked beautiful in the sun. * The well-designed piano sounded lovely. I hope this helped! -Dustin :)