O.K let's make sure you are in the right location. Go to the cave with all of the Octoroks (2nd one) and then go to the top-left of the cave so you can see the corner of that path thing. Then play the Song of Discovery on that corner. Here's a picture:
That's where you play the Song of...
WARNING!! SPOILERS!!!
You can beat Shadow Link in Take 'em All and get a Regal Ring. You can also deliver Dark Ore to Linebek III and he will give you a Force Gem to get to Lost at Sea Station. There is a minidungeon where you can keep beating it and you'll get Regal Rings as the last...