With Master Belch out of the way, the formerly enslaved Mr. Saturn are now free. You and they are rather barftastic after the slog through Belch Base, though, so go through the cave that Belch was guarding, and you'll reach a part of Saturn Valley that you couldn't get to before, because of the short ladder.
Clean Up and have Coffee
In this place, there is a hot springs. Do as the Mr. Saturn says, and "wash barf off you body!" You'll emerge feeling clean and refreshed.
A Mr. Saturn nearby has a cup of coffee for you. Even if you're not a coffee drinker, you should drink this coffee, because it will start a kind of monologue by an unidentified narrator, who reflects on your journey so far and on the things to come.
By the way, you should go back into the cave where the Mr. Saturn told you how to get into Belch Base. There is a different Mr. Saturn in there who will give you some gifts now that you brought Belch down.
Go into the Northeast Cave
When you're ready, go into the cave north of the hot springs. There are some monsters in here (including the humorously-named Ranboob) and you might need to go back into Saturn Valley to get healed up after an encounter or two, because you're at risk of being poisoned, mushroomized, and hurt a whole lot. The hot springs outside of the cave will heal you right up. Once you've leveled up and can handle the monsters here, go all the way through the cave.
There is a Magic Butterfly spawn point somewhere along the way, so be sure to look for it.
At the end of the path, you'll reach Trillionage Sprout. Find out how to defeat Trillionage Sprout. Once it's gone, you can go ahead to Milky Well to pick up the next Sound Stone melody.