This is a marathon question with a likely marathon answer. Bear with me here as I try to answer your question in as short of time as possible.
First and foremost, its pretty obvious to me that you've spent some time thinking about what you want to do, and that you've also spent some time researching the law. Good for you, many do not. But, because your question is so specific, and because there are so many factors that may determine the 'correct' answer, I recommend going over the details with an attorney.
Broadly, the fair use doctrine of the Copyright Act protects the use of copyrighted material for purposes of comment, criticism, parody, etc. But there are a host of other factors that must be considered, including the amount of the copyrighted material used, the nature of its use, commercial impact, etc. You state that there would be no commercial impact. How do you know this? Be very careful when you assume factors in your favor!
So, based upon the facts that you have provided, as well as the law of 'fair use', I'd say you are pushing your luck by posting the entire video game and soundtrack on YouTube, even if you are claiming to comment, criticize, or parody it.
You also run the risk of infringing trademarks (depending on the game posted), which is a separate, but equally important, consideration in itself.
So, repeating my initial advice--speak with an attorney to make sure you know what you are doing.
I am not your lawyer and this comment does not create an attorney-client relationship. Plus, it is not a good idea to base legal decisions solely on comments posted to an open forum board. However, I hope that you will seek the advice of an attorney if you feel that your situation requires such expertise.
To add to my colleague's excellent response, I'd just mention that a "fair use" analysis considers how much of a copyrighted owrk is used, and here, I don't think you'd really ever be able to justify posting the entire game just to be able to comment on it. And if your comments were a pretext for some other use and an attempt to gain a credible defense to a charge of infringement, I think that would be fairly obvious.
Further, credit isn't necessary if a use is truly a "fair" one.
Not seeing your actual use (which is something you need to see your own lawyer to do), I suppose it's possible, but it seems very unlikely.
Avvo doesn't pay us for these responses, and I'm not your lawyer just because I answer this question or respond to any follow-up comments. If you want to hire me, please contact me. Otherwise, please don't expect a further response. We need an actual written agreement to form an attorney-client relationship. I'm only licensed in CA and you shouldn't rely on this answer, since each state has different laws, each situation is fact specific, and it's impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.
One point to add to my colleagues' responses: Whether your posting is lawful as "fair use" is not governed by YouTube's Fair Use Policy but rather the fair use law enacted by Congress and as interpreted by the courts.
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.
Cutting through the red tape, what you're asking for is to do a "Let's Play". LPs are GENERALLY ok, but you MUST be providing commentary, analysis, and other things that shift it into fair user or not. Youtube agrees with this, incidentally -- when I was counsel for a major Tier 1 monetized gaming video site, we had this discussion many times with our video authors and with Youtube. The problem is, that Youtube's takedown process does not factor in their opinion. If a copyright holder wants your video down, it's going down. Then, it is on you to file a counterclaim and risk a lawsuit (which is the rightsholder's next step) to defend your work. Too many DMCA claims, and you lose your monetized status with Youtube, even get your account banned.
Parody is almost certainly not the way to go, as you're unlikely to be considered to have parodied an entire 40 hour game. The specifics of whether any SPECIFIC game/LP is fair use is a legal analysis that we cannot give here on Avvo. If you think you're at risk, hire an attorney.
The author is a Maryland attorney; however no answer given on Avvo is intended as legal advice or intended to create an attorney-client relationship. Dan's expertise lies in the electronic entertainment (video game) industry, as well as complex internet law issues, electronic free speech, entertainment law, copyright and trademark law, and computer fraud. He primarily represents game developers and founders of emergent internet technologies.