Remove the Rabbit Statues
After you get the carrot key from Magnet Hill in Fourside, you can Teleport to Dalaam and open up that trio of rabbit statues standing in front of the entrance to the cave. This will lead you to the next Your Sanctuary.
However, be sure to get the Franklin Badge out of Escargo Express if you don't have it with you. There are a lot of monsters up ahead who use thunder attacks. Give the badge to the weakest member of your party.
Get the Bracer of Kings
You should probably make two visits to the cave. There is a Bracer of Kings in the cave, which is one of the only armor items that Poo can ever equip. However, it's kind of in a dead end of the cave, and once you reach it, you have to go all the way back to the cave entrance to get any closer to the boss.
So you might as well get the Bracer of Kings on your first excursion into the cave (so that you'll have it for fighting the boss), and once you get it, just leave the cave, visit Poo's master to heal up for free, then return to the cave for the actual trip to the cave boss.
Reach the Boss
The cave is fairly straightforward, except for a room with three holes in the ground. If you want to reach the boss, drop down into the far left hole. The other two will drop you off in part of the dead end path that leads to the Bracer of Kings.
Also, after you get the Rock Candy and drop into the hole next to the Rock Candy gift box, DON'T jump into the hole nearby. It's a hole that is on the right side of the room, with no gift box next to it, and no other holes nearby. Don't drop into it, because it will drop you off right next to the Bracer of Kings gift box, which, as I said, is at the end of a dead end.
Instead, go left from that hole, and you'll reach the cave boss. Find out how to defeat Thunder and Storm.
Once you've defeated them, go outside to Pink Cloud to receive the next Sound Stone Melody.