Harmonic Synthesizer

The Harmonic Synthesizer is a treasure in  and. It is a pair of, one blue, the other red. In Pikmin 4, "Melody Rhythm" is written on the blue castanet.

Collecting the treasure in Pikmin 2
This treasure is found in the Glutton's Kitchen's third sublevel, and may be located in an alcove. Breadbugs, Anode Beetles, a Puffy Blowhog, and electrical wires may hinder Pikmin carrying the treasure, so they should be dealt with first.

Note that it is possible for this treasure to not spawn if the cave layout doesn't allow it, as detailed here and here.

Collecting the treasure in Pikmin 4
The Harmonic Synthesizer is near the lake around the center of the Blossoming Arcadia, on the shore to the west. It is guarded by a Yellow Wollyhop, which should be defeated before attempting to collect this treasure, although it is not strictly necessary. To collect this treasure, you must throw 7 Pikmin at it. When you attempt to collect it, a swarm of Mitites will emerge from the ground and induce the fear status in your Pikmin and Oatchi, requiring you to round them all up again.