This is a pretty neat concept. If anything, it's missing the little touches that make the game more engaging. I would have loved to see a score counter. The instructions even ask the player to collect the circle objects, but there seems to be no value to them, so maybe as part of a challenge, they could increase a score value. Another factor of scoring could just be the time of endurance.
Once I got the gist of the game, it did become a little on the easy side, seeing as the only way to die is to touch the black edges, and as you later progress in the game, you tend to get caught in so many rows of moving blocks that it actually becomes quite difficult to die. Seeing as the game is already called "Sound Crush", I guess it would make sense and add an extra challenge if the player could be crushed by the blocks. Well, maybe that would require some difficulty tweaking and definitely a faster respawn time.
The audio is quite cleverly put to use in this, seeing as the moving blocks are synced up to the different cues in the music so as such, it is a neat idea to have the music start out more minimal and then gradually build up as it increases in difficulty. I thought this was a neat idea, and loved watching the blocks line up with the beats and different musical parts.
I know you warned about flashing backgrounds, and they didn't particularly bother me, except for the fact that they don't really seem to match with the music in the game. A flashing background would seem more appropriate if the music was more upbeat and ravelike, but I didn't really get those sorts of vibes from it. So the mood seemed a bit more disjunct.
After the music stopped playing, I was left with a black background and lots of gold orbs. The music started looping again, but no more blocks were coming. Thus, I'm not sure if this was meant to be an endless game? But since the game is dependent on using the music for its challenge, it might have made more sense for it to be beatable (or at least, make it more obvious that it's beatable). Maybe set levels for a postjam version? :D
Overall, I thought there was a great idea and connection with the music and the audio.
~Random-storykeeper