It's pretty interesting, but I'm not very good at it at all. I'm guessing there's some pattern recognition that I need to solve some of these. Also, what do you mean by 'the developers haven't beat all the levels'? Did you procedurally generate the levels and have a function determine how many moves each takes?
It definitely can take a little while to start seeing the patterns and figure out how to solve the puzzles, especially once you get into Advanced Swaps. We are considering rearranging the levels a bit or trying to smooth out the difficulty curve if we can. Once you start to get a feel for things it just clicks and a lot of the early puzzle will be easy, but helping players reach that point is something we are still trying to figure out how to achieve.
Regarding what 'the developers haven't beat all the levels' means, I personally designed all the levels in puzzle mode by hand, but I did make a program that can check what the minimum number of spins required to solve a puzzle is that I can run on each puzzle or on a list of random puzzles (which is used in Arcade Mode and Random Mode). However, the other two people who created the game with me have never used that program and they haven't managed to solve every puzzle so that's what we're referring to.
Some of our nerdy friends are actually trying to come up with algorithms to solve Spinpossible boards (similar to what I wrote as part of our development) and it is a non-trivial challenge.
I like the selection/flip sound effects a lot, but it could use some catchy music as well. Not just one song, though. You could either change the song every X puzzles or give them a list to choose from, because one song reminds me of Marble Blast (sure it was a great song, but for the entire game and all the menus? No), but that's just my opinion anyway.
Yeah my partner did the selection and spin sounds and they came out really well. I think you might be right on music being a good idea. It would give the whole game more of an ambiance. Composing game music ourselves sounds hard, and this is our first game so we don't have a lot of experience with finding music. I'll google around a bit

Thanks for the feedback!