Musical blades of grass. The closer together the blades are placed, the higher the pitch they make; the further apart they are, the grass emanates a lower volume.
The Minotaur were said to have made musical instruments out of them, and to have grazed certain areas of the Plains of Yymp for the specific purpose of creating soothing music. Unfortunately, they also thought Chanteuse Grass was quite tasty, and pleasant sounds made them even more mouth-wateringly attractive. The instruments and soundscapes they created were typically short-lived.
In a passage of EL:MtF, Squeak Dog observes the famous Minotaur musician, Singing Uurhh grazing a particularly soothing symphony from a great expanse of these grasses.