Okay, so what is fair use?

First and foremost, fair use is a defense, not a right. That means nothing is fair use until a judge says it is, and to get a judge to decide that will cost more than is in your bank account. Fair use is a factor test that examines what you are using it for, if you charge, how much of the original IP you are using, and a few other items that all add up together to the decision. So, despite what you read everywhere, IT IS NOT AUTOMATICALLY FAIR USE BECAUSE YOU DON’T CHARGE.

To be clear, I'm not a lawyer (although i could ask one or two )
It is absolutely legal to use samples of music to a certain time limit. I dont know the exact seconds though. The other thing is that youre work is not commercial, you are not earning any money out of it.

And the last point: What could it possibly be that you want to add that is a DIRECT copy of an old game or movie? There are mario maps for nearly every FPS shooter out there. They all had to recreate textures and models themselves because mario is clearly a 2D sidescroller. Nobody can claim rights to that. Movie easter-eggs are also not even a grey zone. What would happen to all the short snippets on youtube or on the internet in general? Its even the opposite, these short clips are really good advertising. In the past companies didnt realize that so they attacked people who clearly advertised for them. The music industry is another problem though. They didnt adjust to the internet as well as the gaming and movie industries. It took them years to understand that people dont want to buy CDs for a horrendous amount of money if they could just download that one song for only 99 cents. The music industry is still a little bit problematic because artists now can live without record labels. They just dont need them because the internet is doin their job.
The only possible problem would be if you really would use assets from an old game in your map but i highly doubt that if you are even worried about music and/or movie clips. If you are really concerned about this I can talk to a friend of mine but i highly doubt you will get into trouble for making easter-eggs. I cant back this up with wikipedia articles right now as Im only remembering some classes i took at the university (EU law btw, so not US).