Flute i believe,you just have to go to echo tower sign and press enter on the sign if it's the echo part,it's south of pororo island where otako is,if you need more hints above that i can say it.
I am not sure about the 2nd last gear but maybe you missed it underwater before entering poseidons domain left the grass or seaweed is stable but behind it it's actually walkable through.
I am actually at chapter 4 now pretty far into it.Looking for the rainbow key and from legendary equipment i only have the sword no other gear though so i wonder when that shows up.
Also what is up with ninja village the contestant is still at the casino even in chapter 4,i only didn't do the jankenpon tournament since i lost,i did win the karaoke one though and got some accesory.