The audio is so bad that I ended up leaving it disabled by default in the settings, so I recommend following the rhythm using the animations as a guide.
Notice those lines that appear on the edge of the screen; every four ticks, one line gets thicker. This indicates the enemies' action time. Defend so you can attack on the next tick.
Maybe I should make a tutorial teaching how to keep up with the rhythm; I think I've been playing so much that I've ended up assuming this kind of thing is basic.