Now, those are some pretty good covers.
Sigma 3: I would say, that it could've been better, but I'm not sure how to improve it... Hmm... I guess the weird zapping percussion thing(hope you know what I'm talking about) could've been on the Triangle channel, instead of the noise. Also, there could've been some chords, whenever you had the chance to use them.
Clock Town: Believe it or not, I haven't played "Majora's mask", so this is the first time I hear this theme (second time was on Youtube, to hear the original) The cover is quite similar, yet it does its own thing sometimes, which is pretty cool. Also, this is a very happy theme, considering there's A GIANT FREAKING MOON ABOUT TO DESTROY EVERYTHING... Oh, wait, the towns are supposed to represent the 5 stages of grief and this is the first one - Denial. So it makes sense.
Arrested Development Intro: FuzzyLizard530 is right. This does sound like the Coin Heaven theme. Can't blame you, though, that's how the notes are
Shia Surprise: Somebody did an 8-bit cover of "Actual Cannibal Shia LaBeouf"... The Internet is a great place isn't it? MechaGrapefruit, I salute you!
Saccharine Dark Lord Astley: Hell yeah! I don't why people complain so much about this song. I know, rickrolling is annoying, but this song is great, and so is this cover. Very nicely done.
So, yeah, the covers are really nice. I would like to hear future covers!
