First of all, I'm going to address the theme song. While it's pretty simple and short, I think it fits the type of series FMTS is very well. I personally think the two lines "a lot of stuff has happened" and "all this crap has happened" are a bit too similar to each other. I think I prefer this version over the one you uploaded to the Audio Portal.
The main issue I have with the animation in this is how the animation sort of moves with the beat. It looks a bit awkward here, though it would probably look worse if he just stood there. It looked sort of alright when he's rapping but when the microphone did it in the beginning it looked a bit uncanny.
The part where he's grabbing for the microphone is pretty good in itself, the only problem is how uneven the line quality appeared to be. At some points it seemed almost squiggly. This tends to happen when I do frame-by-frame, so a lot of times I'll either re-draw it or zoom in and then smooth it out so that it's a more accurate inbetween.
A smaller subject I want to point out is that I think the colors look a little dull in this particular scene. The background is gray and the microphone is a lighter shade of gray, but it still doesn't look very good. There's a variety of ways you can go about this. The most common that I've heard is to make the focus of the shot either noticeably lighter than the background or noticeably darker than the background so that it emphasizes the shot more. You could also blur the background out, but that won't work if there's nothing in the background like in this case. This might be helpful: http://www.colorcombos.com/combolibra ry.html?page=2&search_hex=gray
I'm not a fan of the re-used frames when he's rapping. It reminds me of The Flintstones where they constantly re-use the same motions. I would personally suggest that you vary the timing so that you won't have to re-draw everything, but it still looks fresh.
The animation on the quarters looks pretty good in the beginning, though by the end instead of falling over it looks more like they're shrinking.
The wink at the end was a nice touch.
Overall, it was pretty good but it could use some improvement.