山彦草 / やまびこそう [yamabiko sou] or 'echo herb' in Japanese.
Known as 'Echo Grass' or even 'Echo Drops' in some other translations. Not to be confused with Echo Screen.
Final Fantasy I
Buy: 50 gil (sell: ? gil)
Shop: Elfheim
Drop: Skeleton
Description: Cures silence
Final Fantasy III

Buy: 100 gil (sell: ? gil)
Shop: Canaan, Village of the Ancients, Dwarven Hollows, Amur, Duster, Replito, Saronia NW/NE, Doga's Manor, Ancient Ruins, Invincible, Falgabard
Drop/Steal: Blood Bat, Pugman
Treasure: Tower of Owen, Dwarven Hollows
Type: Consumable
Effect: Removes Silence
Final Fantasy IV
Use: ?
Buy: 50 gil (Sell: 25 gil)
Final Fantasy VI
Use: ?
Buy: ? gil (Sell: ? gil)
Other: can only be used in battle
Final Fantasy IX

Buy: 50 gil (sell: 25 gil)
Shop: Lindblum (Alice's shop, Dragon's Gate merchant, Dragoos' shop (disc 3)), South Gate (Bohden Arch shop, Summit shop), Burmecia (Atlas's mogshop), Treno (slum item shop), Cleyra (Star Maiden Nina's shop), Fossil Roo (Mogki's mogshop, Treasure Hunter's shop, Kuppo's mogshop), Conde Petie (Wendy Grocer's shop), Black Mage Village (item shop), Madain Sari (Morrison's shop), Alexandria (item shop (disc 3)), Esto Gaza (equipment shop (before Desert Palace)), Hilda Garde 1, Oeilvert (Mimoza's mogshop), Desert Palace (Mojito's mogshop), Mt Gulug (Mogtaka's mogshop), Daguerreo (item shop), Ipsen's Castle (Kumool's mogshop), Bran Bal (Moorock's mogshop)
Treasure: (various treasures)
Obtain: dig up during Chocobo Hot & Cold, dig up 5 on the world map with the Chocograph 'Cold Field'
Use: Ether
Description: Cures Silence
Other: as 'Echo Screen', which is in fact another item entirely
Final Fantasy XII
Use: removes Silence from target
Buy: 50 gil (sell: 25 gil)
Other: affected by Nihopalaoa
Final Fantasy Tactics
Use: removes Silence, horizontal range 4 (Chemist ability Item, 120 JP)
Buy: 50 gil (sell: 25 gil)
Shop: (available after Eagrose Castle in ch1) everywhere
Description: Medicine that restores the power of speech to silenced units, allowing them to cast spells.
Category: Item