Last leg and by far the one with the most shenanigans! Flute and bomb, as well as death-resetting will be required for this leg.
Hidden Room 1
Near the head that slants up towards the next checkpoint will be that side path of hillbillies. Run past them (follow the thieving Raccoons) and head the way the screenshots show.
Head to the top and jump back down, heading the bright white path past a few teddies. Toss a bomb at the area shown for a ledge that is a quick path to the Raccoon Kingpen.
Hidden Room 2
Head back to checkpoint (aka die). Right before the first polar bear you run into is an area you can bomb, shown in the screenshot above.
Hidden Room 3
Just the torch vendor's shop!
Hidden Room 4
Just after the checkpoint, where you first use your torch, drop down to collect the yellow bauble and run past the banana. In the screenshots it will lead you to a nice secret shop that will sell you the 750 bauble wallet for 500 baubles.
Hidden Room 5
Get to the top, after the second waypoint, and head past those bananas and spikes. Drop down where the screenshot shows to get in. MAKE SURE ONCE YOU ARE IN YOU ARE PRESSING FORWARD A FEW TIMES SO YOU LAND ON THE PILLAR SHOWN.
Once on the pillar maneuver like the screenshots above shows to net your next location.
Hidden Room 6
Head back to the top, after the second waypoint and head to the left platform of the banana spikes. At the top fall where it shows. And make sure you fall to the LEFT on the "ear rock". At the left play the Gatekeeper's Song to open the mouth for the 3 yellow baubles. DON'T LEAVE!
Hidden Room 7
Head down another level and head to the next mouth shown. Play the Gatekeeper's Song and net yourself a fifty bauble!
Hidden Room 8
Right after the checkpoint where you play a mini-game of Q-Bert. Bomb the wall right across from the opening to get a few baubles.
Hidden Room 9
At the long room filled with spikes and faces on the left and right walls is a spot you can play the gatekeeper's song. Play it where shown. This will lead you to an area which has many frozen waterfall entrances.
Hidden Room 10
Enter at the spot shown and go through the maze like this:
Left, Forward, Right, Right
Hidden Room 11
Which will lead you to a giant area filled with bananas. Lay them all down and you'll open the only mouth in the zone. This will lead you to get the song that shatters ice.
Hidden Room 12-15
So this is a complicated bit of jumping and doing. The video above will explain in detail what is going on here.
Make sure when you are in the final room that you bomb all three walls and go in the RIGHT WALL last (it's a death trap). Also, during the jump path is a secret area that you have to die at, and involved you going left at the first fork in the clouds.
Hidden Room 16
Once you get to the giant puzzle room of spikes and bananas, hug the right wall shown. It'll lead you to a face, play the Gatekeeper's Song and benefit from a shortcut!
Hidden Room 17
Go the mouth shown in the yeti room. Gatekeeper's Song. YOU'RE DONE! YAAAAAAAY!