Your questions and my short answers:
Is using 个 before a noun which has its special measure-word, grammatically correct?
No, it is not grammatically correct.
How would it sound?
It would sound ungrammatical, but most of the time, you would be understood.
Now, the longer answer:
I am assuming that you're asking from a learner's perspective. If you don't know what the measure word for a specific noun is, what do you do? Is it OK to substitute 个? Is it better to leave it out? Is it better to keep silent and wait till you find out the proper measure word?
I believe learning a language is a long process, during which mistakes are made and risks are taken. If you are in a very formal situation, and any kind of grammatical mistake will be highly embarrassing, then perhaps keep silent and and wait till you find out the proper measure word to use.
In everyday life, however, when you're among friends, or friendly strangers, then perhaps treat it as a learning opportunity, use 个 and if possible, ask if that is correct. Using something, even an incorrect measure word, is always better than leaving out the measure word altogether.
Learning the grammar is of course important. But language learning is not just about grammar. It's also about communicability. Communicate, learn, then communicate properly. It's a long and continuous process.
Having said that, many nouns do have semantic cues that can help you make an intelligent guess as to the correct measure word. Take your time, learn those, and build your confidence. Hope that helps.