After receiving the full power of the Sound Stone, Ness senses you must go to Saturn Valley.
Try the Phase Distorter
Find Dr. Andonuts and talk to him to learn about the Phase Distorter. Go into the Phase Distorter. Dr. Andonuts says that you need Zexonyte, which can only be made from material in meteorites.
There is a new Mr. Saturn in the Saturn Valley shop who will sell you pendants and Piggy Jelly. The Piggy Jelly restores about 300 HP. This is not as good as the Kraken Soup, which fully heals one person (except Poo). You can buy Kraken Soup in Summers.
Go to Onett
To get Zexonyte, you should go to the meteorite that crashed into Onett. Teleport there. Visit your house if you wish, then make your way to the hill with the meteorite.
The monsters that you encounter on the way are very dangerous. Ghost of Starman attacks with PSI Starstorm α. Evil Eye can Diamondize members of your party. The Mechanical Octobot can steal from you.
Ghost of Starman has a 1/128 chance of carrying a Goddess Ribbon. If you want it, equip the Rabbit's Foot on Paula and have her use PSI Shield Ω at the start of the battle to block the Starman's PSI Starstorm α. Remember to leave a free space in Jeff's inventory so he can Spy on the Starman to try to steal the ribbon. If Jeff's inventory is full, Spy won't tell you whether or not the Starman has the ribbon.
When you finally reach the meteorite, just check on it to get a meteorite piece. Then use Teleport β nearby to return to Saturn Valley.